A diaper cover provides the waterproof layer needed to keep your child's clothing dry when using cloth diapers. A Diaper cover should be used with fitted diapers.
A pull-on style diaper cover may be used when the cloth diaper underneath also has a closure. If you purchase prefolds, you will need to also purchase a Snappi or pins to make a pull-on style diaper cover effective. A diaper cover that closes with hook & loop style fasteners (like Bummis) will eliminate the need for pins/fasteners when using foldable diapers.