All in one nappies have all the absorbency sewn in, and require the least amount of preparation before use. They come in a variety of fabrics and shell designs. They often have fold out parts to the absorbency to enable a faster drying time. These are the simplest nappies to use.
All in two nappies have an absorbent inner part which attaches to the outer usually by snaps, but can be completely removed for washing. Some designs have outer covers/shells can then be reused, simply snapping in a clean inner. This can make them a more economical option. The inners can usually be tumble dried on low if you wish, too, enabling quicker drying.