| While the term 'string quartet' usually refers to a group of four musicians, it can also be used to describe a style of music (like a concerto or a symphony) written for the aforementioned group of musicians. Music written in this style is, as one might expect, usually written expressly for a string quartet, however it certainly isn't the only style these groups perform.
The string quartet genre became popular during the Classical era. It usually follows a four-movement structure, though Haydn composed some string quartets with five movements during the mid 18th century. The four movements are generally similar to those used in symphonies:
The first movement was in sonata form; it was played at a quick, spirited tempo, and presented a theme or main melody in a variety of different keys.
The second movement was the slower and more songlike aria form. It contained three central sections: an introduction, a contrasting section in another key, and a reprise of the first section.
Much like other classical genres, a string quartet's third movement was based on court dance music. The most popular of these forms was minuet-and-trio, however Beethoven, among others, made use of scherzos and other dance forms. This movement was lively, and almost always in triple meter time.
Though the final movement of a symphony, concerto or sonata could have been sonata form, the fourth movement of a string quartet was usually rondo form. This was a quick-paced movement that contained one main theme played between other secondary themes (as a sort of refrain).
The biggest difference between a string quartet piece and a piece that might be played by a string quartet is the musical structure. Any piece can be arranged for a string quartet, but that doesn't necessarily mean that it's a string quartet piece.
This genre owes its existence largely to the suites of the Baroque period, though it fully developed as its own form during the Classical period. At this point, the suite was considered by many to be old fashioned (its five sections were all played in one key and were all based on court dance music). This is thought to have contributed to the growing popularity of the more varied string quartet.
The string quartet is also considered chamber music, which became incredibly popular during the Classical era. Since chamber music brought music into the homes of the middle class, string quartets were easily accessed by a larger percentage of the population. Because it required only four musicians and their instruments were reasonably portable, this genre was more "user friendly" (so to speak) than full scale symphonies. Rather than going out to the symphony or the opera, members of the middle class could have music brought to them.
String quartet music is written for and performed by two violins (one plays the main melody while the other plays 'filler notes' -- they often switch), one viola (which played harmonies), and one cello. There are several variations on the traditional string quartet, including a string trio which only includes one violin. Other variations on the traditional format include the violins, viola and cello accompanied by several woodwinds, as well as the piano quintet (the four string instruments and a piano).
Since it uses a much smaller group of musicians than a symphony, the string quartet is not as grandiose. It's been described as four part harmony for strings; one (or two at most) of the instruments plays the main melody while the others accent it with harmonies and a bass line.
Resource:
String quartet http://en.wikipedia.org/wiki/String_quartet 1 September 2004
Suite http://en.wikipedia.org/wiki/Suite 1 September 2004
Yudkin, Jeremy. Understanding Music Saddle River, New Jersey: Prentice Hall. 2002. | 
