A string is the vibrating element which is the source of vibration in string instruments, such as the guitar, harp, piano, and members of the violin family. They are lengths of a flexible material kept under tension so that they may freely vibrate. Strings may be “plain” (consisting only of a single material, like steel or nylon), or they may have a core of one material with an overwinding of other materials to increase their mass and thickness. Such strings are called “wound strings.”
The JustStrings website has a glossary covering almost everything imaginable, for classical guitars to basses, from violins to mandolins. They've even got a bit on cryogenic strings.