The viola is a stringed musical instrument which is part of the violin family. In this family it serves as the middle voice, between the upper lines played by the violin and the lower lines played by the cello and double_bass.
The viola is approximately 2 inches longer than the violin (and wider proportionally), and is played in the same manner. Compared to the violin, the viola has a more resonant and mellower sound, especially in the lower registers.
The viola plays notes on the alto clef, and has 4 strings tuned in fifths: C (the lowest), G, D and A - these are tuned exactly one fifth below the violin.