The viola is most easily described as "a slightly larger
violin". Most viola players would argue, though, that the viola is
a very unique instrument with its own history and that a violin is
nothing but "a slightly smaller viola".
Originally, all string instruments were called violas. These instruments came in all sizes (as they still do) and tried to imitate the voices in a choir. Violas were born in the same way as violins; they were created by the Italian Andrea Amati (1511-1577).
As each of the stringed instruments evolved, the viola was almost forgotten in a wave of violin and cello players. Because of its larger size, the viola was clumsier to play and harder to get in tune. In addition, a heavier bow had to be used to produce enough tone out of the thicker strings. Because of these obstacles in becoming good viola players, musicians inevitably chose to specialize in violin or cello instead of viola. With a lack of talented violists, music composers exclusively wrote impressive solo pieces for violin and cello, but not for viola. Even in orchestral works, the viola players usually only got to play uninspired accompaniment to the other orchestra sections.
In this dire period for the viola, interest in playing this seemingly worthless instrument was rapidly declining. Composers were considering abandoning the viola altogether and sticking just to the violin and cello. Suddenly, though, a drastic change brought the viola back to life. With the development of the string quartet by Haydn in the 1760's, there was suddenly a use for the viola. Haydn's string quartets (which called for two violins, a viola, and a cello) were instantly popular and Haydn went on to churn out over 100 of them.
The popularity of the viola gradually but steadily began to improve. Beethoven added to the interest in the viola by writing string quartets in the early 19th century. These quartets involved some difficult passages for viola, but there will still no real virtuoistic viola pieces written. Several composers including Berlioz tried to stimulate interest for the viola in the 1800's by composing viola solos. These generally failed, though, as virtuosos such as Paganini complained that the pieces had "too many rests" and did not have enough impressive passages.
As viola interest once again started to decline, Brahms tried to attract violists by transcribing two of his clarinet sonatas for viola in the late 1800's. These pieces failed, though, because wide leaps that were easy to do on clarinet were almost impossible to do on the viola. Very few solo pieces for the viola were composed in the 19th century and the number of violists was declining rapidly. Then, in the early 1900's an Englishman named Lionel Tertis truly revolutionized the viola as a solo instrument. Tertis quickly transcribed many pieces into viola solos. Among these includes Elgar's Cello Concerto, a very impressive piece on either cello or viola.
Tertis was highly successful in his efforts to bring the viola back to life. One of Tertis' successes was convincing William Walton to write an impressive solo for viola. The viola immediately acheived more fame than ever and attracted the eyes of many composers. A flood of music followed Tertis' efforts written by composers such as Vaughan-Williams, Britten, Bartók, and Shostakovich.
Today, the viola is well established as both a solo, chamber,
and orchestral instrument. Although today's players still
experience the same difficulties with large size, many of the best
viola players -- such as Yuri Bahsmet -- have been recent stars, as
opposed to the virtuosos in the other instruments who succeeded
further in the past.