WBFS, or Wii Backup File System, is a file system developed by Wii homebrew coder Waninkoko. It uses Waninkoko's cIOS and works by creating a WBFS partition on a SD or USB device. A Wii homebrew application may then be run to dump a Wii game to the partition. The titles may then be launched using a Wii USB loader, which uses cIOS and USB 2.0 to load games from an external device.
Although there is no way to natively load a WBFS partition in a PC environment, they can be run in a Wii emulator, such as Dolphin. There are also several applications, such as WBFS managers (Mac OS X, Linux and Windows), that allow the transfer of ISO image files to a WBFS formatted drive. The main advantage of this is the reduction in file size the WBFS format allows, as Wii discs are filled with padding data that must be present in the ISO but that the WBFS file system can strip away. This can allow some smaller games to go from a 4.7 GB (4.37 GiB) ISO file to less than a hundred megabytes as any unused space on the disk will be filled by this padding data.