All Ghouls are creatures infected with a fungal disease - a condition known as Ghoulism. Most ghouls are former Humans, followed less frequently by Manimals. Mutants do not tend to become Ghouls, both because infection is rare given their hardy constitution, and because most Mutants are burned at death. However, there have been very rare instances where a Mutant becomes a Ghoul. Mutant ghouls tend to be very reckless and dangerous. Meanwhile, Botans are immune to ghoul fungus. Ghouls retain much of their physiology from whatever manner of creature they were before they died.
What distinguishes Ghouls from Ghasts and Zombies is the Ghouls' higher level of functioning. Several factors determine what a Ghoul remembers of his first life, primarily how long it takes for Ghoulism to fully take hold after death and the severity of damage caused when the body died. Some ghouls remember large portions of their first life; others remember very little. Regardless, the disease slowly rots their minds just as it does their body. Gaps in comprehension gradually form while the initial holes in their memory grow larger. This typically impacts their understanding of who they were and how to do certain things from their prior life. Some ghouls find this very frustrating and are easily angered. Others recognize and accept it as a consequence of their condition. Some weave confusing and contradictory stories to fill the gaps in their mind, but most recognize these entertaining tall tales for what they are.
Most ghouls live near water or in swamps. The disease compels them to drink fluids far more often than they would have when they were alive.
It is very hard to die again as a ghoul, and most have to wait until the disease runs its course. Ghouls do not age in the same manner as other species. Ghouls simply decay at a slow rate from whatever stage it originally died in its first life (child, adolescent, adult, or elderly). They usually live no more than 15 cycles beyond when they came back from the dead. Their injuries do not heal naturally and wounds are often crudely stitched back together. Any "natural" healing is the fungus replacing things. Therefore, many ghouls take care to avoid altercations. Still others take advantage of this and seek out revenge, or become nigh unstoppable mercenaries - usually taking a great toll on their bodies.
Because of the nature of Ghoulism, ghouls do not have any sort of history. Most ghouls forget more as they age, and can devolve into Zombies or Ghasts.
Most Ghouls have no social culture other than that of their first life. However, due to tensions between races, many ghouls cannot rejoin the cultures they were once part of. More specifically, human-ghouls have the most difficult time dealing with Ghoulism. Most Humans are intolerant of ghouls for fear of contracting the disease, but there are a scant few pockets where Humans and Ghouls coexist.
Ghouls live a very hard life of displacement. They usually join settlements with other Ghouls or Mutants in the wastes. Those that don't usually wander the wastes until their time is up. Most Humans don't want to be near them, and Manimals are very wary, but Mutants are usually very friendly with them. Botans, despite their immunity to the fungus, are openly hostile toward Ghouls.