String instruments are quite self-explanatory: they are defined by the fact that their sounds come from the vibrating strings that are attached to them. In an orchestral setting, sound is produced from an instrument by either plucking the strings or sliding a bow across them, and the instruments in this family include violin, viola, cello, and double-bass. Because hammers strike strings in pianos to create their sound, pianos are sometimes classified as strings. Instruments such as guitars and ukuleles are also technically string instruments, but they are generally not considered in orchestral settings.
Essentials for orchestral string instruments include the bow (horse hair pulled across long and narrow piece of wood for sliding across the strings) and rosin (which creates more friction for the bow). While most bands tune to Concert Bb, strings generally tune to a Concert A.