Creative is a heavily customized Minecraft world. A lot of cheat-commands can be used here, and creative mode is active (but can be switched). Mobs, weather, and almost all automatic events (like spreading of mushrooms) are disabled in this world. You can request to have your building or area protected using WorldGuard so that only you and a few others can build there. Members have build all sort of builds that can be reached with warps.
Vanilla is the survival world on the server. This world has almost the same settings as good old single player, and any command considered a cheat is disabled here. Mobs are on normal mode. Vanilla has an additional plugin called Lockette that allows you to lock your chests from others. Check out the information kiosk at the spawn to figure out how it works.
Flatland is a completely flat world where only donators can build. There's some other features donators can use here, they can all be found on the donating page.
Pixelart is exactly the same as creative, but it's completely flat and meant only for pixelart. Nothing is keeping you from building pixelart in the other worlds, but if you want to show off your pixelart skills this is the place.