The Wii Mini (RVL-201/RVO) is a cut-down version of the Wii system which removes the SD card slot, online functionality, and one USB port. It has a limited software environment removing features related to these related pieces of hardware, such as SD card data management and channels depending on online functionality.
Due to these removed features, the Wii Mini has proven extremely difficult to hack, and while its NAND has been dumped in encrypted form, nobody has yet managed to retrieve the OTP contents (containing the NAND decryption keys) of a Wii Mini.
The Wii Mini features provisions for GameCube ports despite these ports being removed, similar to the Wii Family Edition. It also features pads to install WiFi hardware, and a SD card slot has been successfully soldered on.
The Wii Mini uses modified IOS removing the SD module, and likely WiFi support as well. The Wii Mini uses a modified version of Wii System Menu 4.3 which removes functionality related to online features and the SD slot, similar to the vWii System Menu.