all in one nappies have a waterproof cover built into the absorbent nappy

all in ones generally use laminated PUL polyester for the waterproof outer which may be smooth or topped with velvety minky or cotton 

most all in ones are lined with a stay dry polyester fabric against the skin

the advantage to this system is convenience  while the down side to all in one style nappies is their increased drying time +  cost factor of changing a whole nappy rather than replacing inserts

these disadvantages have been minimised by the innovation of all in two nappies  

all in twos  have removable absorbent inners

removing the inner reduces drying time + means that you can buy more 'inners' and less 'outers' to reduce costs + consumption.