The term rhythm guitar refers to the style of playing guitar chords, and sometimes playing repeated riffs instead of using the guitar to play melody lines or solos (this kind of playing is called lead guitar). Rhythm guitarists add to the sound of a band, whether they play rock and roll, or other kinds of music.
John Lennon of The Beatles usually played rhythm guitar for the band, with George Harrison playing lead guitar. However, Lennon sometimes played keyboards, and once in a while took the lead guitar role, as he did in the songs "You Can't Do That" and "Get Back".
Sheet music for rhythm guitar is usually written above the classical notes. It is written in Guitar chord format.