If you're using a prefold, countour, fitted, AI2, or disposable insert - you'll need a diaper cover. A diaper cover will give you a waterproof layer over a cloth diaper. It will keep the wetness in when using cloth diapers so that it doesn't leak onto your child's clothes. They have a waterproof outer layer that usually consists of fabric that has a PUL (polyurethane), TPU (Thermoplastic polyurethane) backing, fleece or wool.
A diaper cover that closes with velcro style fasteners can eliminate the need for pins or fasteners.
A diaper cover is not necessary if you are using All-In-Ones or Pocket Diapers since those include a waterproof cover.