What does 3G and cellular data do on iphone 4?

What happens if I turn "enable 3G" off?
What happens if I turn cellular data off?

You can't use internet when you have no access to Wi-Fi

If your turn 3G off, then you will have slower Internet connection on your phone. Therefore you might consume less data on your data plan.
If you turn off cellular data, you will have no Internet connectivity at all on your phone. You can only make and receive phone calls. And you can connect to Internet only on Wi-Fi.

Cellular data connects you to the Internet without Wi-Fi. 3G does the same thing, just faster. If you turn of 3G, the Internet will load slower when not on Wi-Fi, however the battery life will be better. If you turn of cellular data, then you will not be able to access the Internet through the cellphone company, you would have to be on Wi-Fi.