Welcome to the NSMBW Knowledge Base. As you might have understood, this wiki is dedicated to storing information related to New Super Mario Bros. Wii modding. This brief introduction precedes a short set of tutorials to get beginners started, and it will try to explain some of the most basic aspects.
Custom levels are levels made by people using a level editor, therefore they are not official add-ons and they're not approved or acknowledged by Nintendo. Whoever claims to have a legitimate licence is blatantly lying.
Hacks are fully-fledged projects which contain a set of custom levels, but also countless other changes such as graphics, soundtrack, worldmaps, and other elements. People usually start by making simple levels, and once they feel they have enough skill, they will start looking into deeper modifications, which can allow you to start your own project once you get to master them.
That said, if you're just looking to play existing mods, check out this tutorial. If you're looking to start modding the game yourself, feel free to go over to the next tutorial to start preparing your modding setup. Enjoy!