That said, plenty of people do it anyway. No matter what, it's a crime to share a ROM with anyone else. However, Nintendo's legal page states that using ROMs of its games for any reason is against the law. If you already own a game, in some cases, you can make a ROM version of it for personal use. While emulators are open source, making them free and legal to use, ROMs are a little more complicated.
You can use the same ROM with multiple emulators, as long as the original game worked with the console you're emulating. A ROM is the software version of a video game. You also need a ROM for the game you want to play. If you want to play a Nintendo DS game, for example, you need to install a Nintendo DS emulator. An emulator is a piece of software that mimics a computer system, like an old video games console.
