A stave is a single horizontal line that is used to represent a single musical pitch. It is the basic unit of musical notation and is used to write melodies and harmonies. When there are multiple staves grouped together, it's called a grand staff or a brace. A staff, on the other hand, is a group of staves that are joined together by vertical lines. This allows for the representation of multiple musical voices or instruments on a single page of music. Staves are most often grouped in pairs (braces) in order to represent the notes played by the right and left hands of a pianist.
In summary, a stave is a single horizontal line used to represent a single musical pitch, while a staff is a group of staves joined together by vertical lines to represent multiple musical voices or instruments.