A string instrument is a musical instrument that makes sound by vibrating the strings on it. There are many types of stringed instruments. The most common of these appear in the violin family. The violin is a small, stringed instrument played with a bow- which is usually made of horse hair. When the bow is drawn across the strings it creates a vibration. You can select what vibration the bow makes by choosing the note on the fingerboard. It is tuned with a peg. The viola is a very similar instrument aside from some small differences. The viola is slightly larger, and is tuned to C, G, D, and A, instead of the violin's G, D, A (440 Hz), and E. The cello is a stringed instrument that is played on the bass clef and is much bigger than the violin, and it is played differently also. The double-bass is bigger still and is the lowest of them all.
There are also plucked strings, the most famous of which would probably be the guitar. The guitar was made famous by rock groups like Led Zeppelin and artists like Eric Clapton. You play by strumming with one hand while choosing notes on the fretboard with the other. Banjos are also very similar.
Here is a very basic list of the stringed instruments: