A staff (pl. staves) is the system of five parallel lines which notes are written on and between.
The pitch range of a staff is determined by a clef which is usually written at the beginning of each musical system.
In American parlance a staff is called a stave.
Figure 1: A basic staff with a treble clef.
A great staff (also grand staff) is a notational device where two staves (one with treble clef and the other bass clef) are brought together and connected by a vertical brace. Between the two staves is the ledger line Middle C.
A great staff is commonly used for piano and harp notation.
Figure 2: A great staff.
Also see System | Brace | Bar.