Body:

The body of the violin is made of two arched plates fastened to a "garland" of ribs with animal hide glue. The rib garland includes a top block, four corner blocks (sometimes omitted in cheap mass-produced instruments,) a bottom block, and narrow strips called linings, which help solidify the curves of the ribs, and provide extra gluing surface for the plates. The ribs are what is commonly seen as the "sides" of the box. From the top or back, the body shows an "hourglass" shape formed by an upper bout and a lower bout. Two concave C-bouts between each side's corners form the waist of this figure, providing clearance for the bow.
The best woods, especially for the plates, have been seasoned for many years in large wedges, and the seasoning process continues indefinitely after the violin has been made. Glue joints of the instrument are held with hide glue, since other adhesives can be difficult or impossible to reverse when future repairs are in order. Parts attached with hide glue can be separated when needed by using heat and moisture, or by careful prying with a thin knife blade. A well-tended violin can outlive many generations of players, so it is wise to take a curatorial view when caring for a violin.

Top
Typically the top (the instrument's soundboard) is made of quarter-sawn spruce, bookmatched at a strongly glued joint down the center, with two sound-holes (or "f-holes", from their resemblance to a stylized letter "f") precisely placed between the C-bouts and lower corners. The sound-holes affect the flex patterns of the top, or table, and allow the box to breathe as it vibrates. A decorative inlaid set of three narrow wooden strips, usually a light-colored strip surrounded by two dark strips, called purfling, runs around the edge of the top, and is said to give some resistance to cracks originating at the edge. It is also claimed to allow the top to flex more independently of the rib structure. Some instruments have two lines of purfling, or have knot-work type ornaments inlaid in the back. Painted-on faux purfling on the top is usually a sign of an inferior instrument. A slab-sawn bass bar fitted inside the top, running lengthwise under the bass foot of the bridge, gives added mass and rigidity to the top plate. Some cheaper mass-produced violins have an integral bass bar, carved from the same piece as the top. Ideally the top is glued to the ribs and linings with slightly diluted hide glue, to allow future removal with minimal damage.

Back and ribs

The back and ribs are typically made of maple, most often with a matching striped figure, called "flame." Backs may be one-piece slab-cut or quarter-sawn, or bookmatched two-piece quarter-sawn. Backs are also purfled, but in their case the purfling is less structurally important than for the top. Some fine old violins have scribed or painted rather than inlaid purfling on the back. The small semicircular extension of the back known as the "button" provides extra gluing surface for the crucial neck joint, and is neglected when measuring the length of the back. Occasionally a half-circle of ebony surrounds the button, either to restore material lost in resetting the neck of an old instrument, or to imitate that effect.
The ribs, having been bent to shape by heat, have their curved shape somewhat reinforced by lining strips of other wood at the top and bottom edges. The linings also provide additional gluing surface for the seams between the plates (top and bottom) and the rib edges.

Neck

The neck is usually maple with a flamed figure compatible with that of the ribs and back. It carries the fingerboard, typically made of ebony, but often some other wood stained or painted black. Ebony is considered the preferred material because of its hardness, beauty, and superior resistance to wear. Some very old violins were made with maple fingerboards, carrying a veneer of ebony. At the peg end of the fingerboard sits a small ebony or ivory nut, infrequently called the upper saddle, with grooves to position the strings as they lead into the pegbox. The scroll at the end of the pegbox provides essential mass to tune the fundamental body resonance of the instrument, and provides a convenient grip for spare fingers to brace against when tuning one-handed, with the violin on the shoulder. Some "scrolls" are carved representations of animal or human heads, instead of the classical spiral volute most normally seen.
The maple neck alone is not strong enough to support the tension of the strings without distorting, relying for that strength on its lamination with the fingerboard. For this reason, if a fingerboard comes loose (it happens) it is vital to slacken the strings immediately. The shape of the neck and fingerboard affect how easily the violin may be played. Fingerboards are dressed to a particular transverse curve, and have a small lengthwise "scoop", or concavity, slightly more pronounced on the lower strings, especially when meant for gut or synthetic strings. The neck itself is not varnished, but is polished and perhaps lightly sealed, to allow ease and rapidity of shifting between positions.
Some old violins (and some made to appear old) have a grafted scroll, or a seam between the pegbox and neck itself. Many authentic old instruments have had their necks reset to a slightly increased angle, and lengthened by about a centimeter. The neck graft allows the original scroll to be kept with a Baroque violin when bringing its neck to conformance with modern standard.

Bridge

The bridge is a precisely cut piece of maple, preferably with prominent medullary rays, showing a flecked figure. The bridge forms the lower anchor point of the vibrating length of the strings, and transmits the vibration of the strings to the body of the instrument. Its top curve holds the strings at the proper height from the fingerboard, permitting each to be played separately by the bow. The mass distribution and flex of the bridge, acting as a mechanical acoustic filter, have a prominent effect on the sound.  Tuning the violin can cause the bridge to lean, usually toward the fingerboard as the tightening of the strings pulls it. If left that way, it may warp. Experienced violinists know how to straighten and center a bridge.

Sound post

The sound post, or "soul post", fits precisely between the back and top, just to the tailward side of the treble bridge foot. Part of adjusting the tone of the instrument is moving the sound post by small amounts, both laterally and along the long axis of the instrument, using a tool called a sound post setter. Since the sound post is not glued and is held in place by string tension and by being gently wedged between the top and back, it may fall over if all the strings are slackened at once.

Tailpiece

The tailpiece may be wood, metal, or plastic, and anchors the strings to the lower bout of the violin by means of the tailgut, nowadays most often a loop of stout nylon monofilament which rides over the lower saddle, a block of ebony set into the edge of the top, and goes around the endpin. The endpin fits into a tapered hole in the bottom block. Most often the material of the endpin is chosen to match the other fittings, for example, ebony, rosewood or boxwood.
Very often the E string will have a fine tuning lever worked by a small screw turned by the fingers. Fine tuners may also be applied to the other strings, and are sometimes built in to the tailpiece. Fine tuners are usually used with solid metal or composite strings that may be difficult to tune with pegs alone; they are not used with gut strings, which have greater flexibility and don't respond adequately to the very small changes in tension of fine tuners. Some violinists, particularly beginners or those who favor metal strings, use fine tuners on all 4 strings. Using a single fine tuner on the E string, or using built-in-tuners, limits the extent to which the tuners' added mass affects the sound of the instrument.

Pegs

At the scroll end, the strings ride over the nut into the pegbox, where they wind around the tuning pegs. Strings usually have a colored "silk" wrapping at both ends, for identification and to provide friction against the pegs. The peg shafts are shaved to a standard taper, their pegbox holes being reamed to the same taper, allowing the friction to be increased or decreased by the player applying appropriate pressure along the axis of the peg while turning it. Various brands of peg compound or peg dope help keep the pegs from sticking or slipping. Peg drops are marketed for slipping pegs. Pegs may be made of ebony, rosewood, boxwood, or other woods, either for reasons of economy or to minimize wear on the peg holes by using a softer wood for the pegs. Attempts have been made to market violins with machine tuners but they have not been generally adopted.

Strings

Violins have four open strings: G, D, A, and E. The four strings run from a tailpiece attached to the base, cross a wooden bridge, continue towards the neck of the instrument running parallel to the fingerboard, and connect to the pegbox located at the very top of the violin. The strings are wound around four tuning pegs that are mounted sideways through holes in the pegbox. The bridge of the violin helps to hold the strings in place, while the pegs maintain the tension necessary to produce vibration.
Strings were first made of sheep's intestines (called "catgut"), stretched, dried and twisted. Contrary to popular belief, violin strings were never made of actual cat's intestines. Plain gut strings are used in both modern and "period" music though in recent years the "baroque" historically accurate performances players seem to use them more often than those musicians who play later period music or play baroque music in a "modern" style. Gut strings are made by a number of specialty string makers as well as some large stringmaking companies.
In the 19th century (and even earlier though not yet prevalent) metal windings were developed for the lower-pitched gut strings. Wound strings avoid the flabby sound of a light-gauge string at low tension. Heavier plain-gut strings at a suitable tension are inconvenient to play and difficult to fit into the pegbox.
There are many claims made that gut strings are difficult to keep in tune. In fact for those who actually have experience with them, plain gut strings are quite stable from a tuning standpoint. Wound gut do have more instability of tuning due to the different response to moisture and heat between the winding and the core, and from string to string. Some players use olive oil on gut strings to extend their playing life, and improve tuning stability by reducing the strings' sensitivity to humidity. Gut strings tend to hold their sound quality nicely right up until they fail, or become excessively worn.
Modern strings are most commonly either a stranded synthetic core wound with various metals, or a steel core, which may be solid or stranded, often wound with various other metals. With low-density cores such as gut or synthetic fiber, the winding allows a string to be thin enough to play, while sounding the desired pitch at an appropriate tension. The winding of steel strings affects their flexibility and surface properties, as well as mass. Strings may be wound with several layers, in part to control the damping of vibrations, and influence the "warmth" or "brightness" of the string by manipulating the strength of its overtones.
The core may be synthetic filaments, solid metal, or braided or twisted steel filaments. The uppermost E string is usually solid steel, either plain or wound with aluminium in an effort to prevent "whistling." Gold plating delays corrosion of the steel and may also reduce whistling. Stainless steel gives a slightly different tone. Synthetic-core strings, the most popular of which is Perlon (a trade name for stranded nylon) combine some of the tonal qualities of gut strings with greater longevity and tuning stability. They are also much less sensitive to changes in humidity than gut strings, and less sensitive to changes in temperature than all-metal strings. Solid-core metal strings are stiff when newly replaced, and tend to go out of tune quickly.
While some gut strings still use a knot to secure the tail end in the slot of the tailpiece, most modern strings use a "ball", a small bead often made of bronze, for that purpose. A frequent exception is the E string, which may be had with either a ball or loop end, since the smallest E-string fine tuners hold the tail of the string on a single small hook.
The price of different string types varies dramatically; gut and gut-core strings are typically the most expensive, followed by leading synthetic core brands, and student steel strings at the lowest price range. Natural gut strings (without the metal windings) are quite inexpensive, especially for the e and a strings. The longevity of strings (all types) is highly variable and is influenced by style of play, chemistry of perspiration and its interaction with the string material, presence of fingernails, frequency of play etc. Some players have trouble with certain brands of strings, or one particular string (say an "a" string of brand xyz) but not with others.
The character of the sound produced by the strings can be adjusted quite markedly through the selection of different types of strings. The most noticeable divisions of sound quality for violins is steel, artificial gut ("perlon" core etc), wound gut, and plain gut. The wound gut tend to have a mellow sound, as do many of the artificial strings, though other artificial core strings are specifically designed to be "bright". Steel and plain gut are both rather bright (full of overtones) but in distinctly different ways: you can tell the difference and yet each are more lively or bright typically than the wound soft-core and wound-gut strings. Certain styles of music have come to be played with certain types of strings, yet there is no hard and fast rule in this respect as each musician is looking for his or her sound. (Country fiddling is often on steel or all-metal; orchestral is often wound (gut or artificial) with a steel e; soloists are often like the orchestra; baroque or early music may be played rather more than romantic pieces, on plain gut.)