When you pluck a string, a note sounds. When a bow is drawn across a string, the result might be a note, but it also might be a hideous screech. There is a fundamental difference between plucked and bowed strings.
As the violinist draws the bow across the strings, it causes the strings to vibrate. Bowing a string transfers energy to it, making the sound richer than the tone of a plucked string. Unlike the staccato sound of a plucked string, a note on a bowed string can last for as long as the bow-stroke continues.
Violin strings vibrate at different pitches depending upon their length and thickness. You can alter the pitch of a string in two ways: by holding it down against the fingerboard or by turning the tuning pegs. The bridge and the body of the violin help transmit the strings' vibrational tones into the air. The F-holes on the front plate allow the body cavity to act as a sound amplifier.