The cello (often called violoncello) is a member of the violin family. It is much larger than a violin, and is also played differently - held upright between the legs of a seated player, resting on a metal endpin on the floor. The cello is played by drawing a bow horizontally across the strings of the instrument, creating a mellow, expressive sound.
The cello has four strings tuned in fifths, and these strings are tuned one octave below the viola. Unlike the other string instruments in the violin family, the cello can play in the treble, tenor and bass clef. The name of the cello itself is derived from the Italian violoncello. It's used very much in classical music, and is part of orchestra, string trio, quartet and other chamber music combinations. Famous cellists include Pablo Casals, Mstislav Rostropovich, Jacqueline Du Pre and Yo-Yo Ma.