Staff paper is usually comprised of staves, which are divided into smaller spaces called measures. The clef being used, as well as the key signature and time signature, are placed before the first measure.
Music for guitar, amongst other instruments, is notated in treble clef. In guitar sheet music, particularly for jazz, it is not uncommon to see chord symbol above measures, as well as what is known as slash notation, which serves to represent a particular rhythm.
Music for piano, or other keyboard instruments, is notated in grand staff, both treble and bass clefs are utilized. The grand staff consists of two staves stacked on top of one another, with the treble clef above the bass clef. Since the piano has such a broad range, one stave does not suffice for notating piano music.
With its roots in the Roman Catholic Church, the art of music notation has undergone many changes, but maintains its central purpose -- to facilitate musicians in the reading and writing of music.
Another name for sheet music is score, usually used to refer to the sheet music of an entire orchestra.