Cellular data network on iPhone?

I turned off the cellular data network option on my iPhone. I'm thinking it means that I can't use the Internet (safari, Facebook, twitter, etc) or receive mms messages and whatnot unless I'm on Wi-Fi. I'm doing this so that my data allowance won't go over.Am I correct about it? And will it keep my data from being
used?

Turn off 3g.

You're correct.