All-in-Ones are the whole diaper in one piece- no stuffing or separate waterproof cover required. With an All-in-One diaper you need a new diaper for every diaper change, there is no reusing any of it until after it's been washed/dried.
All-in-Ones are "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 are best as daytime diapers, as most do not have enough absorbency to last through long naps or overnight by themselves (though they can be made more absorbent with the addition of extra inserts or doublers if needed). However, since they are very user friendly they are great for quick changes on the go, babysitters, or anyone unfamiliar with changing cloth diapers.