Fitteds and pre-folds require covers. Generally speaking, pockets, AIO's (all-in-one - it's just that, 1 diaper, no cover, no insert because the insert is sewn inside; it needs nothing else to work) and AI2's (all-in-two's - has a snap in or lay in soaker which can be washed and dried separately from the rest of the diaper which helps the diaper to dry faster) do not. They have waterproof outers which are most often made of PUL. HTH!
Mama to add 10/05; ds 3/09, and two angels. Expecting my 2nd rainbow baby 2/15!