In Layman's terms, emulators are programs that let you play games designed for a specific console on a different console. Doesn't make sense? Well, this means that, an emulator is a program that will allow you to play a NES game on your PC, or a SNES game on your dreamcast, and so on..... you will need atleast one for each console you wish to use the games on. like one n64, nes, supernes, ect. as for the actual game its not called an emulator, there called roms. so just do a google search, for example n64 roms you will find a bunch of stuff. i like emuparadise.org (sorry if i cant say that here, im new)
yea good explanation on what they are to get them i suggest torrent's if u dont know what torrents are just google it.