-GASP- HOW DARE YOU >:U
Nah, I'm kidding. The difference is the size, the strings, the bow, the clef, etc.
-The viola is bigger than a violin, here's a comparison:

-Secondly, the violin bow has a pointed frog, while a viola has a curved one:

The viola bow is also heavier.
-The viola goes lower than violin, reaching a c-pitch while the violin can only manage a g-pitch. The viola has a C string, whilst the violin, doesn't. In place of that C string, the violin has an E string.
-Violins read treble clef, while violas primarily play alto clef.