Celestia 5e SRD
Ghouls are effectively Zombies which retain some intelligence, though they tend to be feral and animalistic. They seek to consume the flesh of creatures and somewhat ironically eating the flesh of the recently deceased gives them enough energy to remain animated and stave off further rot. However, time between feedings take their toll and eventually Ghouls who are not under the direct control of an animator fall into a ‘stupor’ until they sense the presence of the living. While in Stupor their bodies continue to rot, but at a very slow pace, leaving some to appear as if they were Draugr.
It is said the first Ghouls were accidents in the creation of zombies. However, details are scarce.
Since they do retain intelligence Ghouls actually form social groups. Usually this is fairly tribal, though under a more powerful leader this can take almost any form. Some Corruption Pantheon cities actually have sizeable Ghoul populations.
Powerful Ghouls are often called Ghasts, though they are functionally the same creature.