What's the difference between the pontiac gto & pontiac firebird (or pony car & muscle car)?

im looking to buy a used car and im trying to figure out what is the difference between the two and why they are seperated into two differnt car cles. Im looking to buy a used no older than 1998 and its between a chevrolet camaro, ford mustang, pontiac firebird, or pontiac gto. thanks in advance.