A guitar is a stringed musical instrument. Sound is produced by the vibration of the strings, which in turn resonate through the sound-hole, body and neck (click here for more information info about parts of the guitar).
Guitars may be acoustic, electric (electrical amplification) or both. Classical guitars are also present in the guitar family. Guitars have a body acting mostly as a resonator, which can be hollow in acoustic guitars or solid in most electric guitars, and a neck. Typically, a headstock extends from the neck for tuning.
Strings can be "vibrated" by strumming, or plucking them using your fingers of the hand over the sound hole/pick ups, or a plectrum (pick), shown to your left.
Guitars are used in a variety of musical styles: they are widely known as a solo classical instrument, and are primary in blues, folk and rock.
Thank you, Wikipedia.Org!