How can I figure out where all my hard drive space is going?

Well I have a MacBook with a 160GB hard drive and I have about 20GB left. Now I have been trying to figure out where all that space went, I know I don’t actually have 160GB and it’s around 148GB, but when I get the info from my Users section it says I have about 78GB taken up. I can’t really find out where that other 70GB is taken up, so can someone that is actually an actually an expert on Macs (not swimmindude2496) tell me where that is taken up, or what’s not included under my Users section.