A cello aka violoncello is the bass voice of the string instrument family. It has 4 strings A-D-G-C and it's an octave and a fifth lower than the violin. The cello plays in bass clef and it's the second largest instrument in the string family (the largest is the contra bass/double bass). You use a bow to play or you can pluck with your finger; this is known as pizzicato. The cello is the abbreviated version of the Italian word, violoncello, which means little violone. The violone is the bass. Notable cellists include: Yo Yo Ma, Msistlav Rostropovich, Pablo Casals and Mischa Maisky. If you're interested in listening to some great recordings, listen to the Haydn Cello Concerto No.3 and Six Suites for Unaccompanied Suites, by Bach (especially the Bourree from the 3rd Suite; Rostropovich's recording on YouTube is awesome!)
The viola on the other hand is the alto voice of the string family. It is larger than a violin. A 3/4 size viola is approx. the size of a 4/4 violin. The viola has four strings A-D-G and C and is a fifth lower than the violin. The viola can be played with a bow or by pizzicato.The viola's tone ranges between the violin and the cello but the low and rich sound sets it apart from the high pitched graceful sound of a violin. The viola is played identically to the violin.
The 'cello is larger than the viola, which is larger than the violin.
ViolinViolaCelloBass
A cello can play in the range of a violin (much of it), if that's what you mean, but a violin's highest note is much higher than a cello's highest note.
The viola is both larger than the violin and smaller than the cello.
violin, viola, (violin) cello, (double) bass
The 'cello is larger than the viola, which is larger than the violin.
1st violin 2nd violin Viola Cello
ViolinViolaCelloBass
The cello.
Typically a string quartet is two violins, one viola, one cello.
viola
A cello can play in the range of a violin (much of it), if that's what you mean, but a violin's highest note is much higher than a cello's highest note.
You hold a cello between your knees, whereas you hold a viola/violin between your chin and shoulder.
A viola is a string instrument of the violin family. It is larger than a violin but smaller than a cello, and like the violin and cello it is played with a bow.
The viola is both larger than the violin and smaller than the cello.
violin, viola, (violin) cello, (double) bass
The 'cello is the larger of these instruments.