As far as homebrew goes it'll keep any IOS you've installed. HBC will be gone of course so if you're on 4.0 you'll have to use bannerbomb or whatever the current exploit is (I don't particularly care or keep up since I have no reason to look into the current exploits of getting HBC on there now) if you want to get HBC back.
As far what formatting will do it'll pretty much wipe all what you're looking for it to. VC, WiiWare, any other channels you've installed will be wiped along with any tickets associated with them. Game saves will be wiped. Pretty much everything of that nature. Your Wii Shop channel account will remain though (use for your purchase records so you can still re-download your legally purchased VC and WiiWare) unless you choose to delete that manually as well.
So pretty much apart from installed IOS's (ios 249, etc...) everything will be wiped.
