A photographic
print made using
light cast directly through a sheet of
negatives onto a piece of
photo paper.
What you end up with are a whole bunch of
little-bitty pictures the size of the negatives you just used.
Contact sheets are only really useful for one purpose: deciding which of your negatives are worth printing afterall.
If you're lucky enough to have your own darkroom and do all of your own printing rather than sending it off to a lab, a contact sheet is usually the first step taken soon after developing film.