The guitar is a popular musical instrument classified as a string instrument with anywhere from 4 to 18 strings, usually having 6. The sound is projected either acoustically or through electrical amplification (for an acoustic guitar or an electric guitar, respectively). It is typically played by strumming or plucking the strings with the right hand while fretting (or pressing against the frets) the strings with the fingers of the left hand. The guitar is a type of chordophone, traditionally constructed from wood and strung with either gut, nylon or steel strings and distinguished from other chordophones by its construction and tuning. The modern guitar was preceded by the gittern, the vihuela, the four-course Renaissance guitar, and the five-course baroque guitar, all of which contributed to the development of the modern six-string instrument.

There are three main types of modern acoustic guitar: the classical guitar (nylon-string guitar), the steel-string acoustic guitar, and the archtop guitar. The tone of an acoustic guitar is produced by the strings's vibration, amplified by the body of the guitar, which acts as a resonating chamber. The classical guitar is often played as a solo instrument using a comprehensive finger-picking technique. The term "finger-picking" can also refer to a specific tradition of folk, blues, bluegrass, and country guitar playing in the United States.

Electric guitars, introduced in the 1930s, use an amplifier that can electronically manipulate and shape the tone. Early amplified guitars employed a hollow body, but a solid body was eventually found more suitable, as it was less prone to feedback. Electric guitars have had a continuing profound influence on popular culture.

The guitar is used in a wide variety of musical genres worldwide. It is recognized as a primary instrument in genres such as blues, bluegrass, country, flamenco, folk, jazz, jota, mariachi, metal, punk, reggae, rock, soul, and many forms of pop.