Instead of a separate fitted nappy and cover system,  All In Ones simply have the waterproof cover built into the absorbent nappy. 

The advantage to this system is convenience.
All in ones generally use polyester for their waterproof outer, whether that be the smooth finish of PUL, or the cuddly finish of  minky fabric. 

Most all in ones are lined with a stay dry fabric against the skin. 

Most all in ones come in different sizes. 

The disadvantage of all in one style nappies is drying time and the cost factor of having to buy bulk quantities in different sizes over time.

These disadvantages have been minimised by the innovation of All in Twos which are sometimes also called "almost in ones" or "AI2s". 

This style of nappy has removable absorbent inners-removing the inner helps reduce drying time, amd means that you can buy more 'inners' and less 'outers' to maximise your investment in this style of nappy.  The itti bitti d'lish snap in one and the Berry Plush are good examples of this style, using bamboo for the absorbency.

New in store is the new bumgenius All in One. This nappy is a true all in one with a twist....drying time is mimimised and greater flexibility achieved through the inclusion of an optional pocket style opening so that extra absorbency can be added.

Also recently released is the new bumgenius organic nappy, which is a true all in one that is very cleverly designed to fit right through to toilet training, plus it  uses certified organic Egyptian cotton against your baby's skin. Snap button fastening style.