A guitar is a string instrument. There are many types of guitars, such as bass, acoustic, electric, multi-neck, 6-string, 12-string, etc.
In 'The Beatles' there were 3 guitarists, John Lennon who was rhythm guitar, Paul McCartney who was bass guitar and lead guitar on songs such as Drive My Car, Taxman, Paperback Writer, Sgt. Pepper's Lonely Hearts Club Band (song), and Helter Skelter and George Harrison who was lead guitar. The most famous guitar was Paul McCartney's bass.