Mammatus are pouch-like cloud structures. They're also a rare example of clouds in sinking air-- most clouds form in rising air. Although mammatus most frequently form on the underside of a cumulonimbus, they can develop underneath cirrocumulus, altostratus, altocumulus and stratocumulus.
For a mammatus to form, the sinking air must be cooler than the air around it and have high liquid water or ice content. They derive their name from their appearance, like the bag-like sacs that hang beneath the cloud resemble cow's udders.
Mammatus often forms in association with Cumulonimbus clouds, which in turn bring thunderstorms due to their huge mass of unstable air. Mammatus cloud generally form in the most unstable cumulonimbus, meaning that there is also a chance of hail, heavy rain and lightning in the vicinity, and if the air is cold enough during winter they can produce snow.