In addition to varying in levels of absorbency, diaper covers run the gamut in style, from basic to fancy: Some are pull-ups, while others fasten with snaps or Velcro or Aplix tabs, and they come in lots of colors and patterns.
Not all cloth diapers require the use of a separate cover. Some of the styles that do need a cover include fitted diapers and prefolds.
When the weather is warm enough, many parents let their babies bum around in diaper covers; this also makes for quicker changing. You'll probably want to have five or six. They can be reused two or three times before they need to be thoroughly cleaned; just wipe the inside with a damp cloth. Prices start at about $8 and top out at $18 or so.
