Although cats are extremely agile, there are areas of their bodies which their tongues simply cannot reach, including their ears, nose, the back of their heads, and the area around their mouths just outside reach. For these locations, they use a front paw as a sort of a "washrag." They first dampen it by licking it, then wipe the moisture from the paw onto the area to be cleaned. They will perform this ablution several times in each area. Finally, when they are finished, they will clean the foot they used.
The next time your cat eats, watch him closely and you will witness the entire operation. Washing up after eating is another lesson we humans can learn from cats.