Ghouls are former humans who have become horribly irradiated by the radioactive fallout covering the wasteland from the Great War. Many ghouls are pre-War humans who survived the initial nuclear bombardment during the Great War. Becoming permanently irradiated, they were horribly disfigured and their lifespans were increased virtually indefinitely. In most cases, exposure to radiation in the post-War Commonwealth wastes degenerated their brains, causing them to lose their higher cognitive functions, turning them into mindless, radiation-resistant cannibals.
The brain structure of a feral ghoul indicates that the regenerative ability of the neurological system that affords "normal" necrotic post-humans their longevity does not extend into the higher reasoning functions of the brain itself. This condition is referred to as "ferocious post-necrotic dystrophy," according to the ghoul physician Barrows. Despite the apparent lack of the higher reasoning they once had as humans, some are capable of semi-intelligible speech, as the Vault 34 security officers will occasionally growl out understandable phrases similar to the trogs of The Pitt, indicating that their vocal chords and their brain's speech centers are still relatively intact.
Dr. Banfield supports the theory that all ghouls will eventually become feral ghouls however, it should be noted that he has never taken the time to actually study a ghoul and is biased against them. It should also be noted that many of the ghouls in existence were born prior to the war and have yet to go feral making it quite possible that this theory is inaccurate.
As with normal ghouls, ferals are not only extremely resistant to radiation, they are actually healed by it, as can be seen when a glowing one uses its radiation blast nearby other ghouls. M.A.R.Go.T. also states that feral ghouls have no internal body heat.
It is now possible to dismember ghouls, however they will continue to chase and bite the Sole Survivor even if they lose both of their arms. When losing a leg, however, they will become docile and will no longer try to attack, allowing the player to quickly dispatch them and handle any uninjured ghouls. Rarely, a feral ghoul will still attack after its head is decapitated. It is unknown if this is a bug.
A notable difference between the Massachusetts feral ghouls and feral ghouls seen throughout New California, the Mojave Wasteland, and the Capital Wasteland, is that they have glowing yellow eyes. Sometimes feral ghouls will lie on the ground, either to rest or play dead. Once the player character gets close enough or they are otherwise alerted, they get up and attack. These ghouls can sometimes be targeted in V.A.T.S. and if spotted, attacked before they rise. They also show up as living actors on the HUD if the Sole Survivor is wearing power armor with a targeting HUD-equipped helmet or aims at them with any kind of Recon Scope.
Occasionally, they will start to run faster at the player moving erratically to dodge incoming attacks, suggesting some form of intelligence, and will eventually lunge at their attacker once they're close enough. A gurgling roar is heard when a ghoul charges at the Sole Survivor. This attack can sometimes be difficult to dodge without sprinting, but it can however be blocked or interrupted with a melee strike. Additionally, when feral ghouls lunge at the player, they often fall down beside or behind their target, giving the player a brief chance to strike while the ghoul is getting back to its feet. In some locations, particularly indoors, ghouls will also emerge from windows, ductwork, etc., to attack when a victim comes close. Ghouls hiding in this sort of ambush cannot be detected even by V.A.T.S. before they begin emerging - however while they are emerging they are helpless while climbing into the room and can be freely attacked if one is quick. Often they will not detect the player until they finish emerging.
They very rarely travel alone, and it is quite unusual to encounter a single ghoul. This behavior has lead to groups of ferals often being called "packs" or "hordes" by wastelanders.
Another notable difference from previous versions of ferals is that they inflict radiation damage as well as physical damage.
Stalkers are a variant of feral ghouls that can easily be distinguished by their bloody mouths and paler skin. They often pretend to be dead or hide amid rubble until an enemy is detected, where they then attempt to quietly ambush their prey without revealing themselves, usually with success. There are generally at least a few stalkers mixed into larger groups of weaker ferals. As they wear the same tattered clothing as roamers and normal ferals, they can be difficult to spot until they're damaged. They will also sometimes serve as a "boss" in locations populated by ferals. If the player is of a high enough level, the boss may instead be a glowing one, which the stalker will still accompany.
Reavers usually have pieces of post-war metal armor attached to them as in Fallout 3's and New Vegas', possibly implying these are rather recent victims of ghoulification. As in New Vegas, they lack a projectile attack, and are generally the ones in a pack to reach better-equipped targets thanks to their respectable amount of HP. They are much easier to deal with than past versions. Unlike the ones in Fallout 3 and New Vegas, their corpses do not emit green fumes.
Withered feral ghouls are slightly skinnier, with a yellowish leathery-looking skin, and wear no clothes. These ghouls are likely fairly old compared to others, as their clothing has rotted away and they appear to be severely sun-damaged.
A withered feral ghoul can be found inside the power armor storage in the national guard armory. Their eyes and mouth also appear severely damaged and shriveled.
Gangrenous feral ghouls are considerably rarer and stronger than the previous variants, with torn rags like common variants, and yellow skin. They can be hard to defeat even for some mid/high level player due to their aggressive attacks and high health. A gangrenous feral ghoul can be found inside Croup Manor's basement guarding a chest with some expensive loot. One can also be found at O'Neill Family Manufacturing in the Glowing Sea.
Rotting feral ghouls have a bloated appearance, indicating that their bodies have begun decomposing, also suggested by the name. Although their offensive capabilities are on par with gangrenous feral ghouls, they have almost twice the amount of health.
The highest tier of all non-glowing feral ghouls, this variant is covered by a thick layer of rust and is much tougher to kill than a normal feral. These level with the player character, making them immune to the Wasteland Whisperer perk.
A highly irradiated version of a feral ghoul, they glow with green light emitting from within, and just being near them causes radiation damage. They can perform an area of effect radiation attack indicated by the raising of their arms which releases radiation around them. This radiation also heals nearby feral ghouls, and glowing ones can even sometimes revive recently killed allies. They are potentially more dangerous than past iterations, especially against low-level players, due in part to their radiation-fueled healing, although their passive radiation radius is smaller and thus they are easier to dispatch without taking any radiation.
A higher level version of glowing one, covered in glowing green blisters of radiation. Its radiation aura is almost twice as strong, but it is no longer capable of performing its area of effect radiation attack. This is the strongest type of feral ghoul in the game, damage-wise.
A higher level version of glowing ones. Its body is bloated and swollen with radioactive material. It gives off more radiation than its counterparts and the melee attack is a little weaker, compared to a putrid glowing one. However, it does have the highest health of all feral ghouls.
A variant only found on the Island and mutated by the island's radioactive fog. Fog ghouls are much paler than ghouls found in the Commonwealth, with black burns on certain parts of their skin and fishing nets draped over their chest. Their eye sockets are quite large, with the eyes sunken in, and their teeth are permanently gritted, exposing the gums, which seem inflamed, and the teeth sunk in as well. Black soot is also smeared across their faces.
Feral ghouls encountered in Suffolk County charter school are a unique shade of pink not seen in any other population.
Feral ghouls are not hostile towards cats. This might be due to the game's data which does not allow the Sole Survivor to interact with cats and, as a consequence, also does not allow other characters to do so as well. 
Feral ghouls' limbs are severed when broken, unlike other creatures.
Feral ghouls can continue to survive, even if (the top part of) their head has been blown off. However this is extremely rare and this may even be a bug.
Feral ghouls will randomly become friendly if one has the Ghoulish perk.
The Children of Atom believe that feral ghouls were originally ghouls but rejected Atom and became feral. 
Feral ghouls can be found in most graveyards in game.
The only way to prevent a ghoul from being reanimated by a glowing one is by blowing its head off or destroying its torso (using the Bloody Mess perk).
Ferals can sometimes do a finishing kill on both the player (in third person mode) and NPCs by jumping on top of its victim and biting their neck.