Sadly, it's not a hard drive like many people assumed. Instead, you'll be able to directly download games to SD cards. At the moment, the only way to get games on SD cards is to download them to your Wii first and then transfer them to the SD card, meaning you have to create space first, but with an update coming in the spring (At least for Japan), this will no longer be necessary - Just download them straight to the SD card. Of course this still means you'll have to swap games, so it's not really that great of a solution - Oh well!
Note: We originally said games would be playable directly off of SD cards. Sadly, this is not true - Our apologies!
Source: Nintendo of Japan