Original: It consists of four main parts (precincts), of which only the largest, the Precinct of Amun-Re, currently is open to the general public. The term Karnak often is understood as being the Precinct of Amun-Re only, because this is the only part most visitors normally see.
Simplified: The precinct of Amun-Re is the only part of Karnak which is open to the public.