All-in-Ones are the whole diaper in one piece- no stuffing or separate waterproof cover required. All-in-Twos are a two piece diaper system, which consists of a reusable waterproof cover and an absorbent snap-in insert. They are both "diaper" shaped, have enclosed elastic at the legs and back, and close with either snaps or hook&loop (like Velcro). In general, all-in-ones/twos are best as daytime diapers, as most do not have enough absorbency to last through long naps or overnight. However, since they are very user friendly they are great for quick changes on the go, babysitters, or anyone unfamiliar with changing cloth diapers.