What are the files that are gone when we format our wii?
savedata?
channel?
mii?
ios?
system firmware?
cios?
trucha?
homebrew installed application?
When you format the Wii, all the downloaded channels (Internet Channel, Nintendo Channel, Homebrew Channel, etc.) and save data will be deleted from your console. It will not "downgrade" your System Menu, or change any of your IOS versions in any manner. It basically will wipe the memory clean, and make it as if you just turned it on for the first time.
Source: | Nintendo - Customer Service | Wii - Data Management (http://www.nintendo.com/consumer/systems/wii/en_na/data.jsp#format)