Covers/wraps are a separate waterproof cover that can be used with fitted and prefold nappies. They come in PUL, wool or fleece. PUL is the most popular choice for day and night as it is easy to care for, maintenance free and very trim. PUL is short for polyurethane laminate - basically a polyester fabric is laminated to make it waterproof and prevent leaks.
Wool or fleece covers are better options for overnight, over a fitted nappy. The covers are breathable, absorb moisture and prevent leaks. Fleece is inexpensive and can just be put in the washing machine with your nappies. Wool covers can be aired unless soiled as they have a ‘self-cleaning’ ability. They do require lanolising to maintain their waterproofing however they have superior moisture absorption and leak prevention than fleece covers.