Crepe is a very general term for any fabric with a crinkly surface - achieved by a combination of the way it's woven, with some threads having floats of 3 or 4 threads instead of the plain one-over, one-under weave, or can be combined with a satin weave on the other side. The threads from which crepe is woven are highly twisted which makes them want to curl up and twist back in the other direction.
It can be made of any fibre type, and any weight, so georgette is a very lightweight crepe, crepe de chine (any fibre, not just silk) is light weightcrepe backed satin in usually mid range, and I think that triple crepe is the heaviest.