Like every main mario game,SM3DU comes with worlds,but this time it contains 12 regular worlds and 6 secret worlds,each which contain levels
Regular levels (usually indicated by 1,2,3 and so on) contain no boss battles and are the most common levels,they always contain a flag at the end and 3 crystal stars.
Junior Castle Levels
Junior Castle Levels (indicated by a smaller version of castle icon) usually contain boss fights (usually Bowser Jr. but sometimes the Koopalings are boss fights) and are usually a bit harder then regular levels,though they still contain 3 crystal stars.
Castle Levels (indicated by an icon of a castle) usually contain boss fights (sometimes Bowser) and are alot harder then Regular Levels,they contain 3 Crystal Stars and one Bowser Gem.
Toad Levels (indicated by a Toad icon) are where you can play as Captain Toad,these are very different from other levels and contain more then 3 crystal stars usually,only the final Toad level has a boss fight.
Koopa Levels (indicated by a Koopa Shell) is where you race Koopa the Quick,there is one crystal star here and no boss fights at all.
World 1 is the easiest world,being a mostly grassy world with the exception of Bowser's castle which contains lava.