Classic Wiki

The viola is the middle-range instrument of the violin family. It is sometimes cavalierly referred to as the "big fiddle." Its position in the violin family somewhat parallels the alto voice of the normal SATB (soprano, alto, tenor, bass) arrangement in a choir of voices, the alto being just below the soprano range. As do other members of the violin family (violin, cello, contrabass) the viola has four strings, the lowest of which descends at an interval of a fifth below that of the violin.

The viola is played with a bow and placed on the shoulder, as is the violin, in contrast to the cello, which is placed between the player's legs. The etymology of the word viola, or viola da braccio, leads some historians to believe that when the violin family emerged as an entity in Italy during the early part of the sixteenth century, the viola may have appeared slightly before the violin, violino being a diminutive form of viola. Violists often like to think that they may indeed have been at the head of the family, at least historically.

Source The Viola website 

The viola is an instrument in the violin family. It is slightly larger than the violin with a deeper, mellower tone. It’s tuned a fifth lower (although it is hard to distinguish from a violin when played in an orchestra, unless it’s on the C string). It parallels the human alto voice. In early orchestra music, the viola’s role was merely harmony. It often had the bass line when the composer couldn’t think of anything else to write. The melody was left to the brighter, higher violin and more powerful, distinct cello. Into the 20th century, the viola began to get a more prominent role, and is now sometimes featured as a solo instrument.

Source essortment