What is a necropolis?

I am happy with this for now... I think I heard of this before but I was never sure what it really meant. the word came up again in a recent Discovery Channel TV Show, about Ancient Greece.

The Greek word meaning; city of the dead normally describes large and important burial areas that were in use for long periods of time.

http://en.wikipedia.org/wiki/Necropolis

The Wikipedia page lists over 20 examples all over the world of different Necropleis. I've been to one of the frour they list for Mexico. When I was younger I got to visit Teotihuacan, and all the pyramids there and the temple as well. I never realized (until now) that this was a cementary. Duh! what are pyramids usually for? burial grounds right? Should have guessed that.