Infinite dungeons are dungeons that are created as they are delved into, and once someone leaves it- it is destroyed, until someone goes to enter it again, each room is created as the room currently made is explored.
Rooms deeper in these 'infinite dungeons' are much more dangerous, and in turn provide more loot, slowly going from fighting lesser enemies to monsters such as zombies, wendigos, and other such monstrosities.
The loot in these dungeons is also mostly randomly created- however some items are personally created by... Something, within the dungeon.