Coinciding with the launch of the Wii U Animal Crossing: New Leaf community, the plaza is a place your Mii can go to keep track of game news, stay up-to-date with special in-game calendar events, and share their sketches, screenshots, and custom clothing designs with the rest of the world. Players will be able to upload their 3DS shots and custom creations via SD card, organizing them into albums. QR codes for player-created content can be scanned directly off the screen.
Animal Crossing Plaza should be available now (I've not seen it yet) as a free Wii U download. The service will run through the end of 2014.
Update: The app is now available as a 146MB download.