The golem is a creature of Jewish myth; a man formed of clay, as Adam was said to have been, and brought to life by a skilled rabbi. This usually involves a holy word, either written on parchment and placed in the golem's mouth or carved directly into its forehead. The resulting golem works as a servitor, and is incapable of speech. In the most famous version of the tale, it eventually goes berserk when its creator forgets to deactivate it on the Sabbath, and it ends up breaking the holy commandment not to work at such times.
In D&D, however, the word is used for a much larger class of creatures, humanoid forms made of pretty well any material and animated to serve their creator. Some of these, too, have a habit of going berserk, although the trigger for doing so is decidedly more random.
Clay Golem
The clay golem is, of course, the one that most closely resembles the being in the original myth. In this respect, it's notable that it has to be created by a powerful priest (and, in, 1E, specifically a priest of a benevolent deity), rather than a wizard or necromancer. In 1E, it is shown as a humanoid figure sculpted from clay and apparently wearing a loincloth. For all we can tell from the picture, this might be a part of the moulded clay, just an artistic convention by its creator, but, from 2E onwards, we're told that a leather jockstrap is an essential accoutrement for all clay golems.