
A clone of Ibudan in an early stage of growth
A clone is an organism that is genetically identical to its ancestor. Clones could be produced via asexual reproduction, or through technology. Though usually an exact duplicate, some clones were modified to differ from their ancestor.
There were also natural ways to create a clone. The Lyssarian Desert Larvae for example, when injected with DNA from another organism, was transformed into a mimetic simbiot which was identical to the original donor and also carried his memories. In 2153, Charles Tucker III had one such clone, Sim. (ENT: "Similitude")
The Dominion were experts in cloning, which they used to create successive copies of individual Vorta. As of 2375, Weyoun had been cloned seven times. (Star Trek: Deep Space Nine) The Vorta cloning process was described by Weyoun 7 as a very delicate one. Once in a while, a clone could turn out "defective", resulting in untypical behavior, like treachery. (DS9: "Treachery, Faith and the Great River")
In 2155, the xenophobic Terra Prime group created a binary clone of T'Pol and Charles Tucker III, deliberately inserting genetic flaws in an attempt to "prove" that Humans and aliens should not associate. She was named Elizabeth, but she died in her infancy due to flaws in the cloning process. (ENT: "Demons", "Terra Prime")
The Mariposa colony was inhabited entirely by clones of the five original settlers. Since the colonists knew that two women and three men could not sustain a society, they looked to science to survive and turned to cloning. Future generations later referred to them as the "progenitors". However, by the 24th century the colony's survival was threatened by their limited genetic diversity because of a phenomenon known as replicative fading. In 2365, the USS Enterprise-D introduced new genetic material to Mariposa in the form of the colonists of Bringloid V. (TNG: "Up The Long Ladder")

Spock with Spock Two
In 2269, Stavos Keniclius 5, himself a clone, created a giant clone of Spock called Spock Two on Phylos. Spock Two was several times bigger than the original Spock. Keniclius intended to create a master race of peacekeepers based on Spock's genetic template, but abandoned the idea when he learned that peace keeping was not necessary in the Federation. (TAS: "The Infinite Vulcan")
By the 24th century the aspect of cloning seemed repulsive to some Humans, considered to eliminate the aspect of one's uniqueness and specialty. When offered the chance to be cloned, William T. Riker claimed that a hundred or a thousand of Rikers would diminish that status. (TNG: "Up The Long Ladder")

Picard and his clone, Shinzon

Jean-Luc Picard and Shinzon's DNA match, establishing that the latter is a clone of the former
Circa 2364, the Romulans created a clone of Jean-Luc Picard, named Shinzon, as part of a plan to infiltrate the Federation. Shinzon's RNA incorporated temporal RNA, so that his growth could be accelerated, but they were never activated. The plan was abandoned when the government changed. Much later, Shinzon would become Praetor of the Romulan Star Empire and plan an invasion of the Federation. (Star Trek Nemesis)
In 2369, Ibudan created a clone of himself and killed it in order to implicate Odo. However, his plan was discovered and he was arrested for murder, as under Bajoran law killing one's own clone is an equal crime to other forms of homicide. In the meantime, a second clone of Ibudan was created in Bashir's infirmary and went on to make a new life for itself. (DS9: "A Man Alone")
The Kobliad Security officer Ty Kajada ruled out the possibility that Rao Vantika was a clone after she studied the DNA reference scan analysis. (DS9: "The Passenger")
In 2376, at the First Annual Voyager Science Fair brothers Azan and Rebi presented cloned potatoes. They originally had the idea to clone Naomi Wildman but Seven of Nine suggested that they try and clone something smaller. (VOY: "Child's Play")