A guitar is a musical instrument with a long neck and strings. When you play the guitar, you press the strings with one hand to make a note, and strum with the other hand (unless, of course, you are playing air guitar).
The section of a guitar where your fingers form a note is called the "fingerboard." An acoustic guitar has a hollow body that vibrates with warm sound when you pluck or strum the strings, and an electric guitar is solid and plugs into an amplifier that projects the sound. The word guitar comes from the Spanish guitarra, which has its roots in the Greek kithara, a lyre-like stringed instrument.