Was is a file type Like mp3 its just an extension like iso And the Wad installer is the thing that installs it onto ure wii (as a channel)
And yes u can get ure twilight back if u save it onto ure computer
Heres a link to the wad installer
GBAtemp.net :: Downloads :: Other useful tools :: Wad Installer 2.1
Now Extract the files onto the root of the Sd Card And rename the elf file(wad-installer.elf) to boot.elf
Then
Create a folder called "wad" in the root of the SD card.
2. Copy every WAD you want to install in the "wad" folder.
3. Run The Application