Oxygen in the air combines with the metal (mostly lead) of the battery post and clamp. It will be worse if the connection isn't good because it will get hot, and if there are acid fumes from the battery vent. Try recleaning the clamps and posts with a mixture of water and baking soda, then reconnect. Some suggest that you coat the connections with vaseline or even grease, however this will collect dirt.
Your battery may be leaking. If you've already cleaned the terminals thoroughly once, and the corrosion comes back, it's probably time to replace the battery.
Also check to see if your alternator is overcharging the battery- this can cause boiling that will lead to corrosion on the outside of the battery case.
It's a good idea to remove the sheetmetal support, soak it in a baking soda / warm water solution, and paint it with a good quality paint. While you're at it, you might want to clean and paint the plenum area where the support sits, since acid can leach into these seams and cause rust. I've had good luck with POR.