Infdev is a mode still under development, in which the playing area is said to be several times larger than the surface of the Earth. Currently, it generates strange landforms, including floating islands and strange looking cliffs that do not form normally in indev. If you die, you can respawn at your starting point, but you drop all of your items you were carrying and can pick them up if you go back to where your body is. Mobs have not yet been added, along with a few more key features from indev, although Notch intends to readd everything in upcoming updates.
Sometime in the future, indev will become Infdev, and indev save files (mclevel) will not work.
You must buy the game in order to play infdev.