The viola is the second smallest instrument of the string family. You all know the violin, the cello, and the string bass. Ever wonder why they have string quartets instead of trios? It's because there's another string instrument. The viola. It looks just like a violin, except it's a little bigger. As for the difference in sound, the strings of the violin are tuned to E, A, D, and G. The strings of the viola are tuned a perfect fifth lower--A, D, G, and C. Also, the viola tends to have a fuller sound than the violin, more like a cello. Personally, I like it better than the violin--but that's just me personally. I'm a violist. :)

Here are some MIDIs featuring the viola in small string ensembles:

String Quartet No. 2 in Dmaj (3rd movement), Alexander Borodin
Air and Variations (The Harmonious Blacksmith), George Handel
String Quartet in Cmin, ?

For those of you who recognize Borodin's String Quartet as "And This Is My Beloved" from the show Kismet, you should. Wright and Forrest, the writers of Kismet, used some of Borodin's themes for their music. Another example is Borodin's "Polovetzian Dances", the theme of which was used for Kismet's "Stranger In Paradise".