The pitch of a sound is determined by the frequency of the vibrations, while the timbre of a sound is determined by the shape of the vibrations. The louder the sound, the more energy is being transferred to the air by the vibrations.
Strings can produce a wide variety of sounds, depending on the material they are made of, the tension they are under, and the way they are plucked or bowed. For example, steel strings tend to produce brighter sounds than nylon strings, and strings that are under more tension produce higher-pitched sounds.
Stringed instruments are some of the oldest and most popular musical instruments in the world. They are used in a wide variety of genres of music, from classical to folk to rock.