The violin is by far the most common orchestral instrument. It is also the smallest out of all the strings and has the highest pitch. This is usually the first choice of musicians who want to learn a string instrument (unless they have viola, cello, or bass friends).
The viola is slightly larger than the violin and has a deeper pitch in comparision. It is also in the "middle register" of the string family. Although not as popular as the violin, it is still a vital part of the orchestra.
The cello is still bigger than the violin and viola. It has a beautiful, mellow sound. The cello must rest on the floor because it is too big to be held like the violin or viola.
The bass is the biggest instrument in the string section. It also plays the widest assortment of music from classical to broadway to jazz. Although it appears the same size as the cello in the picture, it actually about 1 1/2 to 2 times taller.