Mud has been one the cultural mainstays of humanity since prehistoric hunter-gatherers smeared colored dirt on their faces and danced around a bonfire and chanting to ward off evil spirits. This practice is not in general use in modern society, although there are many people who encrust their faces with specialized mud produced in factories in an attempt to attract members of the opposite sex.
Another popular use of mud is mud pies, which are treated in their own very fine article.
MUD, is an acronym, standing for, at varying times Multi-User Dungeon, Multi-User Dimension, or any of a number of variations on this. A MUD is a place on the internet where people all around the world converge and go around an imaginary world killing monsters, or each other, or any combination of the two, or just talking. There are many, many MUDs out there. There are several types of MUD, LPmud, Diku, and Circle are examples of more combat-oriented types, while various MUSH's, MOO's, and MUCK's tend to be more social, although these distinctions are not etched in stone.
Mud, is a glorious substance, ask any pig, rhinoceros, elephant, or other large mammal and they will, assuming that you can find one that talks, expound on the glories of a good wallow. Mud keeps the sun off, provides protection from insects, and is useful when stung by one of the more pointy bugs. It can provide camoflage, or it can just be a source of entertainment, and, as any rose-bush can tell you, mud just feels nice between the toes.
Mud is such a major part of our culture that there are infinite used for it. For more information on this and related subjects, visit your friendly neighborhood local library.