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.