Minecraft wiki entry for Custom_maps, associated text: Survival Maps. Like regular Minecraft survival, but with a twist. You may be in the clouds, have limited resources/space, etc.
CTM maps. Short for "Complete the Monument", these maps are known to be very difficult. The goal is to complete a "Victory Monument", where you have to fill it up with wool, records, or some other type of items. These maps have many traps along the way.
Arena Maps. These maps have stages and in each stage, you must fight a wave of mobs in order to progress.