To add: GPRS, is a 2G medium. It has a slower transfer rate than 3G, but consumes less energy. That's why your iPhone's battery will last longer if you disable your 3G and run on GPRS instead.
Both GPRS and 3G make up your mobile data plan, although you will, of course, need more time to download stuff using a GPRS protocol compared to a 3G one. Both will ultimately add up to be the same amount of data to your data plan.