Musical clef is a symbol which indicates the pitch of notes written on the staff. It defines one reference note (G, C or F) and assigns it to a specific line of staff.
G-clefs has reference tone G4 placed on the line that goes through the curl of the clef.
F-clefs has reference tone F3 placed on the line that goes between the two dots of the clef.
C-clefs has reference tone C4 placed on the line that goes through the center of the clef.