Ghoul

Ghouls (グール,) are zombie-like vampires that are created when a vampire drains the blood of a non-virgin human. They're classified as a more powerful creature than a zombie, yet far from a vampire.

Appearance
After being reanimated, Ghouls appear sickly and grey-skinned. Their teeth are sharp and elongated, with the Millennium variation bearing fangs very similar to a vampires. The eyes of a Millennium Ghoul are also purple, to the pale orange of a normal ghoul. Their bodies also appear to be rotted and quite vulnerable to tearing.

General
They are under the control of the vampire who bites them, eat human flesh, and are intelligent enough to use firearms. Due to their undead state, ghouls possess low-level invulnerability to conventional injury; allowing them to survive multiple gunshot wounds and even being dismembered. They are however, vulnerable to weapons made of silver; especially when hit in the head or heart. When fatally wounded, they instantly crumble to dust. It is not rare to see a vampire amass a small army of ghouls for offense and defense. Despite their relative skill compared to the lumbering undead of other lore, they are still quite slow and lack the intellect to execute any manner of defensive maneuvers, making them vulnerable to specialized weaponry. Once they have been tainted, ghouls cannot be cured, and can only be saved by ending their suffering.

In anyway, they're not smart; in fact, they can be considered brainless. They're just a moving corpse however, with a far superior durability and strength than that of even the strongest humans. Despite this fact, they're slow and that's what makes them so easy yet sometimes, so hard to kill; because of the fact that creatures like this are kept secretive, so normal humans meeting them would easily freak out. That is one of the advantages ghouls get, despite being easily killed by experts and such. Ghouls are also considered a "failures who've unsuccessfully attempted to become full-fledged vampires", but they're still monsters to be feared.

Millennium Ghouls
Ghouls, as suggested by Walter C. Dornez and Jan Valentine in Volume 2, seem to not only follow orders to fight, but comply with any other orders from their vampiric masters as well. Artificial vampires can make all of their victims ghouls regardless of the status of their virginity. These types of Ghouls also survive even if the host vampire is destroyed. Altogether, ghouls created by the Millennium Organization are of a much higher quality than regular ghouls.

Fate of the Soul
The victim's soul is taken in by the vampire if they are turned into a ghoul (although a soul can be taken if they're not directly bitten, such as licking blood from a bleeding body, or if the victim is dead when they're bitten, regardless of their virginal status). It is possible that the ghouls can still manifest, even if it's original body is destroyed, as long as the vampire that created them is still in possession of their soul, as Abraham Van Helsing claimed to have destroyed all of Dracula's servants and yet he was still able to manifest all the soul's he'd taken, when at release state level 0; and later claimed he still had to destroy all the souls inside him, despite Anderson destroying their manifestations in the London invasion. Once the host vampire who made them destroyed, the souls they've taken will be destroyed as well.

Known Incidents Involving Ghouls

 * "Cheddar Incident" - Created from the original citizens of the town and Seras Victoria's fellow police officers by the Cheddar Priest. Most were killed by Alucard during his battle with the vampiric priest, and the rest died with their master when Alucard killed him.
 * "Badrick Incident" - Created from the original citizens of the town by an unknown artificial vampire. Killed by Alexander Anderson, Seras Victoria, and Alucard.
 * Attack on Hellsing HQ - The Valentine Brothers brought an army of ghouls equipped with heavy riot gear for their attack on Hellsing Manor, but the army was defeated by Walter C. Dornez and Seras Victoria. During the attack, the invaders also turned most of Hellsing's security personnel into ghouls, but they were torn apart by a berserk Seras before being put out of their misery by Sir Integra Hellsing.
 * The Eagle's Hijacking - Created from the ship's crew by three vampiric traitors among them. It is unclear if the advance agents were artificial vampires or natural vampires that simply allied with Millennium, but all the ghouls were quickly killed along with their masters when Rip van Winkle and the Nazi soldiers hidden in the Eagles cargo hold took control of the ship.
 * London Invasion - Created from London's citizenry by the Letzte Bataillon. Killed by Integra Hellsing, Walter C. Dornez, and the Papal Knights of the 9th Crusade, and also by the organization that did this themselves.

Trivia

 * The third episode of the original series, it's implied that even if a vampire doesn't kill a non-virgin with their bare hands (such as using guns) the dead will still become ghouls.