LOLOLOLOL, OK well I’m not a Mac expert now (Expert is actually a specific title within the store, haha), and I’m actually doing in-depth software and product training this week, but this is something that I kind of already know a bit about anyway.

No, upgrading to a new OS doesn’t automatically wipe everything on your harddrive, not even on a PC unless you’re doing a clean install. BUT you would be crazy to not back up your files first in case something happens to go wrong. It could potentially happen, and you could potentially lose everything. DEFINITELY back it up first, and it’s a good thing to do anyway regardless in case your hard drive dies on any random day. This has a few different ways of doing that (Apple-approved). I highly recommend Time Machine because it’s the easiest, fastest, and cheapest, and you already have the program built-in to OSX, probably (depending on your version). This is a pretty easy guide to what to do before installing Lion. I hope that helped! If you feel like you’re not ready to do it yourself or anything, definitely go into a store and make an appointment to get someone to help you out!