The difference between a violin and a viola, is that they do have different strings. The violin has the E, A, D and G string. The viola does not have the E string, but it does have the C string. So, it goes A, D, G, C is the lowest string; whereas, A is the highest string for the viola. And on the violin, the E is the highest string for violin, and the G is the lowest string on the violin. So, that's one difference.
The other difference is that they also play from different clefs. The violin is in the treble clef, and the viola is in the alto clef.
Another difference is the sound quality. The viola has a real mellow sound to it, a little deeper; whereas, the violin has a higher, sweeter, high-pitched sound.
So, those are the differences between the violin and the viola.