Fitted diapers are an absorbant diaper that fits all the way around your baby, or in other words the entire diaper is absorbant. Fitted diapers require a cover to make them waterproof or to stop leaks, but with newborns some babies can go without a cover for the first couple weeks as they are not wetting that much. They fasten with either velcro or snaps that are attached to the diaper itself.
Contours are similar to fitteds except they do not have a fastener to hold them shut. When using a snug cover or wrap you do not need to fasten the contour because the wings will stay in place from the cover/wrap holding them; this allows a slightly quicker diaper change. Some parents still choose to use a snappi or pins to hold the contour shut.
When paired with a wool cover both fitteds and contours are a good night time system or for a heavy wetter because there is so much more absorbant material.