Cloning is the process of creating new individuals (clones) that are genetically identical to other individuals, or otherwise creating new individuals by using the genetic material of one or more template individuals.
Within the Gargoyles Universe, cloning is most often carried out by the geneticist Anton Sevarius, who frequently clones gargoyles. Notable clones include Thailog (a clone of Goliath), as well as Burbank, Malibu, Brentwood, and Hollywood (who were cloned from other members of the Manhattan Clan). Delilah, though not technically a "clone" in that she is not a genetic copy of any one individual, was nevertheless "cloned" from the genetic material of Demona and Elisa Maza combined. These clones, with the exception of Thailog, joined the Labyrinth Clan in October 1996, and are occasionally referred to collectively as the Clones.
In 1994, after the Manhattan Clan refused to work for David Xanatos, Xanatos hired Anton Sevarius to produce more gargoyles (and gargoyle-like beings) to replace them. One such strategy that Sevarius employed was to clone Goliath, using genetic material obtained by Owen Burnett. The project was a success and the clone, named Thailog, was artificially aged to the prime of his life (equivalent to a human in his mid-twenties) in a special tank. During this aging process, Thailog was programmed with Xanatos' mindset. The accelerated aging process also altered Thailog's skin and hair pigmentation.
After being released, Thailog quickly turned on Xanatos and stole $20 million. He fled to Europe and later began a relationship with Demona, the former mate of his genetic original, Goliath. Together they set up their own company, Nightstone Unlimited. ("Double Jeopardy", "Sanctuary")
At some unknown point, Thailog came up with the idea of creating more clones that he could use as "flunkies". When he pitched the idea to Demona, she went along with it, partly out of a desire to have a new clan of her own.  The two gargoyles returned to New York City and hired Sevarius. Demona allowed herself to be taken prisoner, then used a series of robotic mosquitoes to obtain the DNA necessary for their new creations. The mosquitoes collected samples from Hudson and the Trio, then delivered them to Sevarius at the Nightstone Unlimited office building. From these samples, Sevarius created the clones known as Burbank, Malibu, Brentwood, and Hollywood. At Thailog's secret request, without Demona's knowledge, he created Delilah from the combined DNA of Demona and Elisa.
The Clones were artificially aged, like Thailog, to bring them into the prime of life equivalent to a human in their mid-twenties, thus making them a little older than the trio, and much younger than Hudson, and generally much tougher when they were pitted against the Manhattan Clan at Coney Island.  This rapid aging also accounts for their odd skin and hair pigmentation Cloning a clone's DNA and rapidly aging it would likely create a pigmentation and appearance that wouldn't look exactly like the original donor, but it would look similar).  At Sevarius' advice, Thailog kept their programming simple, merely instructing them to obey him. Ultimately, nothing is wrong with the clones' minds, they just lack education. 
The Clones were eventually subdued after Demona and Thailog were (incorrectly) presumed dead in a roller coaster fire. Talon decided to take them under his case at the Labyrinth, and the Clones joined the Labyrinth Clan as its original gargoyle members. When Thailog resurfaced on Halloween, they opted to remain at the Labyrinth with the exception of Brentwood, who chose to join Thailog. ("The Reckoning", "Bash")
Behind the Scenes
According to Greg Weisman, there were three main inspirations behind the creation of the show's first clone, Thailog: a mix session during the making of the 35mm movie version of "Awakening" where Greg noticed Elisa saying the word 'Thailog' while the name 'Goliath' was being rewound, John Byrne's color change of the Fantastic Four's costumes upon their return from the Negative Zone, and the desire to see Keith David's talents work double duty as hero and villain.  Darkwing Duck's Negaduck was also a likely visual influence. 
Further, while the episode "Double Jeopardy" intentionally misdirected the audience in its first act, Greg Weisman writes that he did not want the episode to ultimately be "EVIL TWIN takes the place of the good guy and confuses everyone".  The John Byrne-inspired pigmentation change would also serve to solve another one of Greg Weisman's pet peeves, the tendency for a clone to miraculously be the same age as their genetic original.