Mammatus are very "saggy", bag like clouds. These clouds usually form after a bad thunderstorm has passed by. The difference between Mammatus and most other clouds is that this specific type of cloud forms in air that is sinking, as opposed to most other clouds, which form in air that is rising. When the cloud forms, the sinking air must be cooler than the air surrounding it.
Mammatus Clouds are sometimes described to look like the udders of a cow...
These clouds also look like Marshmallows, making the name Mammatus easier to remember.http://eo.ucar.edu/webweather/cloudpic9.html
http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/cld/oth/mm.rxml
http://www.flickr.com/photos/rjmcconnell/3674141460/ First pic
