The viola is not the best known instrument in the violin family. In fact, ask anyone you know that is not involved with orchestra what it is and they would blink dumbly at you. In reality, even though the violin is better known in today's society, it is possible that violas appeared before violins because the Italian word for violin, violino, is derived from the word viola, although no one knows who invented it or when it was invented.
To answer anyone's question on what exactly a viola is, a viola is the alto in the violin family, which evolved from the viol, an instrument with many strings that is bowed and held across the knees. They used to be called the "alto-tenor" violin. The viola we know today is the result of the gradual merging of the alto and tenor violas over three centuries. Most musicians did not like to play the tenor viola because it was so large in size and difficult to play because of this.
So, gradually, luthiers stopped making the tenor viola. The alto viola's neck was too short to play the music that was getting more difficult by the 16th century. So luthiers lengthened the neck of the alto viola in order for musicians to play a wider range of notes. The modern-day viola is about 1-3.5 inches longer than a violin, making them around 16 to 18 inches long. Today they are the only instruments that use the alto clef.
Violas are not nearly as famous as the violin or cello, and they probably never will be. There are very, very, few solo violists, especially compared to the amount of solo violinists. In the ordinary orchestra, there are about three violins to every viola, and sometimes more. Composers sometimes wouldn't even bother...