(From 5th Edition Monster Manual - 2014):

With their razor-sharp teeth and jagged claws, ghouls roam the night in packs, driven by an insatiable hunger for humanoid flesh.

Ghouls roam the night in packs, driven by an insatiable hunger for humanoid flesh.

Devourers of Flesh. Like maggots or carrion beetles, ghouls thrive in places rank with decay and death. A ghoul haunts a place where it can gorge on dead flesh and decomposing organs. When it can’t feed on the dead, it pursues living creatures and attempts to make corpses of them. Though they gain no nourishment from the corpses they devour, ghouls are driven by an unending hunger that compels them to consume. A ghoul’s undead flesh never rots, and this monster can persist in a crypt or tomb for untold ages without feeding.

Abyssal Origins. Ghouls trace their origins to the Abyss. Doresain, the first of their kind, was an elf worshiper of Orcus. Turning against his own people, he feasted on humanoid flesh to honor the Demon Prince of Undeath. As a reward for his service, Orcus transformed Doresain into the first ghoul. Doresain served Orcus faithfully in the Abyss, creating ghouls from the demon lord’s other servants until an incursion by Yeenoghu, the demonic Gnoll Lord, robbed Doresain of his abyssal domain. When Orcus would not intervene on his behalf, Doresain turned to the elf gods for salvation, and they took pity on him and helped him escape certain destruction. Since then, elves have been immune to the ghouls’ paralytic touch.

Ghasts. Orcus sometimes infuses a ghoul with a stronger dose of abyssal energy, making a ghast. Whereas ghouls are little more than savage beasts, a ghast is cunning and can inspire a pack of ghouls to follow its commands.

(From Monster Manual v3.5 - 2003):

This foul creature appears more or less humanoid, but has mottled, decaying flesh drawn tight across clearly visible bones. It is mostly hairless and has a carnivore’s sharp teeth. Its eyes burn like hot coals in their sunken sockets. 

Ghouls haunt graveyards, battlefields, and other places rich with the carrion they hunger for. These terrible creatures lurk wherever the stench of death hangs heavy, ready to devour the unwary. Ghouls are said to be created upon the death of a living man or woman who savored the taste of the flesh of people. This assertion may or may not be true, but it does explain the disgusting behavior of these anthropophagous undead. Some believe that anyone of exceptional debauchery and wickedness runs the risk of becoming a ghoul. The transformation from living beings into fell things of the night has warped their minds, making them cunning and feral. Ghouls speak the languages they spoke in life (usually Common). 

Combat: Ghouls try to attack with surprise whenever possible. They strike from behind tombstones and burst from shallow graves. 

Ghoul Fever (Su): Disease—bite, Fortitude DC 12, incubation period 1 day, damage 1d3 Con and 1d3 Dex. The save DC is Charisma-based. An afflicted humanoid who dies of ghoul fever rises as a ghoul at the next midnight. A humanoid who becomes a ghoul in this way retains none of the abilities it possessed in life. It is not under the control of any other ghouls, but it hungers for the flesh of the living and behaves like a normal ghoul in all respects. A humanoid of 4 Hit Dice or more rises as a ghast, not a ghoul. 

Paralysis (Ex): Those hit by a ghoul’s bite or claw attack must succeed on a DC 12 Fortitude save or be paralyzed for 1d4+1 rounds. Elves have immunity to this paralysis. The save DC is Charisma-based.

LACEDON:

These cousins of the ghoul have the aquatic subtype. They lurk near hidden reefs or other places where ships are likely to meet their end. They have a base land speed of 30 feet and a swim speed of 30 feet and are found only in aquatic environments. 

GHAST:

Although these creatures look just like their lesser kin, they are far more deadly and cunning. 

Combat: Ghoul Fever (Su): Disease—bite, Fortitude DC 15, incubation period 1 day, damage 1d3 Con and 1d3 Dex. The save DC is Charisma-based. 

Paralysis (Ex): Those hit by a ghast’s bite or claw attack must succeed on a DC 15 Fortitude save or be paralyzed for 1d4+1 rounds. Even elves can be affected by this paralysis. The save DC is Charisma-based. 

Stench (Ex): The stink of death and corruption surrounding these creatures is overwhelming. Living creatures within 10 feet must succeed on a DC 15 Fortitude save or be sickened for 1d6+4 minutes. A creature that successfully saves cannot be affected again by the same ghast’s stench for 24 hours. A delay poison or neutralize poison spell removes the effect from a sickened creature. Creatures with immunity to poison are unaffected, and creatures resistant to poison receive their normal bonus on their saving throws. The save DC is Charisma-based.

(From Monstrous Manual - 1993):

Ghouls are undead creatures, once human, who now feed on the flesh of corpses. Although the change from human to ghoul has deranged and destroyed their minds, ghouls have a terrible cunning which enables them to hunt their prey most effectively.

Ghouls are vaguely recognizable as once having been human, but have become horribly disfigured by their change to ghouls. The tongue becomes long and tough for licking marrow from cracked bones, the teeth become sharp and elongated, and the nails grow strong and sharp like claws.

Combat: Ghouls attack by clawing with their filthy nails and biting with their fangs. Their touch causes humans (including dwarves, gnomes, half-elves, and halflings, but excluding elves) to become rigid unless a saving throw versus paralyzation is successful. This paralysis lasts for 3-8 (2+1d6) rounds or until negated by a priest.

Any human or demi-human (except elves) killed by a ghoulish attack will become a ghoul unless blessed (or blessed and then resurrected). Obviously, this is also avoided if the victim is devoured by the ghouls. Ghoul packs always attack without fear.

These creatures are subject to all attack forms except sleep and charm spells. They can be turned by priests of any level. The magic circle of protection from evil actually keeps ghouls completely at bay.

Habitat/Society: Ghouls and ghasts are most frequently encountered around graveyards, where they can find plenty of corpses on which to feed.

Ecology: Ghouls (and ghasts, as described later) delight in revolting and loathsome things — from which we draw our adjectives “ghoulish” and “ghastly”.

Lacedon

The lacedon is a marine form of the ghoul. Lacedons are sometimes found near marine ghosts, particularly ghost ships. Lacedons are less common than ghouls because of the fewer corpses available for them to feed on, but they can often be found swarming around recent shipwrecks in rivers, lakes, and oceans.

Ghast:

These creatures are so like ghouls as to be completely indistinguishable from them, and they are usually found only with a pack of ghouls. When a pack of ghouls and ghasts attacks it will quickly become evident that ghasts are present, for they exude a carrion stench in a 10’ radius which causes retching and nausea unless a saving throw versus poison is made. Those failing to make this save will attack at a penalty of -2.

Worse, the ghast shares the ghoulish ability to paralyzation, and their attack is so potent that it will even affect elves. Paralysis caused by a ghast lasts for 5-10 (4+1d6) rounds or until negated by a priest’s remove paralysis spell.

Ghasts, like ghouls, are undead class and thus sleep and charm spells do not affect them. Though they can be struck by any sort of weapon, cold iron inflicts double normal damage. Clerics can turn them beginning at 2nd level. The circle of protection from evil does not keep them at bay unless it is used in conjunction with cold iron (such as a circle of powdered iron or an iron ring).