What is an emulator?  What is a ROM?

An emulator is basically like a video game console on your computer, and a rom is basically like a video game on your computer. 

How can I use emulators and ROMs? 

If you wanted to play Super Mario Bros. for the Nintendo Entertainment System, you could download the Super Mario Bros. ROM file and download a Nintendo Entertainment System emulator (but first see our disclaimer for legal details).  You could then open the emulator and run the ROM file in the emulator.

Is downloading ROMs legal? 

Read our disclaimer.

Why isn't my Super Mario Bros. ROM working?

If your Super Mario Bros. ROM is starting at level 0-1, you need to use a different NES emulator.  The only one that I know will definitely work is VirtuaNES.  This principle also applies to hacked Super Mario Bros. ROM.

Which emulators are the best?

I usually put the best emulators at the top of the page.