Fitted diapers are cut to fit the shape of a baby without any folding needed. The term "fitted" refers to the elasticized fit around baby's legs - you get a much tighter and more leak-proof fit than with prefolds. Fitteds can be sized, or they may have rise snaps to make them "one-size". Fitted diapers are not waterproof, so a separate cover is needed.
Many parents find that fitted diapers make a great overnight cloth diapering system, because the entire fitted diaper is absorbent all around, rather than just the smaller rectangular area of absorbency found in a pocket diaper or AI2.