The viola is the alto-voiced cousin of the violin. It looks very much like a violin, to the point where it can be difficult for the casual viewer to tell the difference, until he sees it next to a violin. It is not, however, a violin. It is larger and more mellow-sounding, and - most unlike the violin - there is not a standard size for the full-sized viola. It is between 1-4 inches longer than the violin. Some manufacturers have tried making them larger to improve the sound; others, shorter, with an eye toward making them more ergonomically correct.
Here is a video featuring both the viola and the violin in a way that you can really see and hear the difference. It is Mozart's Sinfonia Concertante, 1st. mvt., performed by Francesco Manara (violin), Simonide Braconi (viola).