The Homebrew Channel is a channel for Wii that was not made by Nintendo and is not supported by them. This channel is a freeware program that can be used to launch unofficial games and programs, as well as progams that can bypass the regional lockout or use normal DVDs (for program running; movies will not work on Wii console made after a certain year because DVD commands were removed). Other programs can load full games from a USB device, or launch normal games with cheats.
The channel was originally accessed by downloading a modified save file from The Legend of Zelda: Twilight Princess and playing the file. By moving one step in the game, it sideloaded the channel to the Wii's memory. As of System Menu 4.0, the Twilight Hack has been blocked. It has been replaced with bannerbomb, which works on System Menu versions 3.2 to 4.3; letterbomb, which works on System Menu 4.3; and Smash Stack, which works on all System Menu versions but requires Super Smash Bros. Brawl. The Homebrew Wiki is here.
Downloading the channel will install an invisible bug, called the Trucha Bug, and void your Wii's warranty.