Nintendo roms?

are the emulators and roms illegal to play? I downloaded it a few weeks back and then I found out on an official nintendo website that roms and emulators are illegal. Even though I see people use it on youtube and stuff and the interent let’s you download certain content like “Super Mario World” and stuff like that, I don’t see how it could be illegal to own. as long as you don’t sell it to other people.
thnx man, I was saying the same thing to my dad, but he won’t belive me….