Emulation is The process of simulating hardware of one machine on another, so in this case, playing SNES, Genesis, N64, etc. games on your PC, Mac or mobile device
The image of the actual game; for example: Super Smash Bros. This is a file that has been converted from a game cartridge or CD so that you can run it on your computer or mobile device
A program that allows you to run SNES, Genesis, N64, etc. game images (ROMs/ISOs) on your computer or mobile device
A list of Emulators can be found here
A list of ROMs/Games can be found here by selecting a system/emulator. Once a system/emulator is selected, all available ROMs will be shown.