Cloning is the process of creating an identical copy of an original. A clone in the biological sense, therefore, is a single cell (like bacteria, lymphocytes etc.) or multi-cellular organism that is genetically identical to another living organism. Sometimes this can refer to "natural" clones made either when an organism reproduces asexually or when two genetically identical individuals are produced by accident (as with identical twins), but in common parlance the clone is an identical copy by some conscious design. Also see clone (genetics).
The term clone is derived from κλων, the Greek word for "twig". In horticulture, the spelling clon was used until the twentieth century; the final e came into use to indicate the vowel is a "long o" instead of a "short o". Since the term entered the popular lexicon in a more general context, the spelling clone has been used exclusively.
In biology, cloning is used in two contexts: cloning a gene, or cloning an organism. Cloning a gene means to extract a gene from one organism (for example by PCR) and insert it into a second organism (usually via a vector), where it can be used and studied. Cloning a gene sometimes can refer to success in identifying a gene associated with some phenotype. For example, when biologists say that the gene for disease X has been cloned, they mean that the gene's location and DNA sequence has been identified, although the ability to specifically copy the physical DNA is a side-effect of its identification.
Cloning an organism means to create a new organism with the same genetic information as an existing one. In a modern context, this can involve somatic cell nuclear transfer in which the nucleus is removed from an egg cell and replaced with a nucleus extracted from a cell of the organism to be cloned (currently, both the egg cell and its transplanted nucleus must be from the same species). As the nucleus contains (almost) all of the genetic information of a lifeform, the "host" egg cell will develop into an organism genetically identical to the nucleus "donor". Mitochondrial DNA, which is not transferred by this process, is generally ignored as its effects on organisms are thought to be relatively minor.
The term clone is used in horticulture to mean all descendants of a single plant, produced by vegetative reproduction. Many horticultural varieties of plants are clones, having been derived from a single individual, multiplied by some process other than sexual reproduction. As an example, some European varieties of grapes represent clones that have been propagated for over two millennia. This is a genuine example of cloning in the broader biological sense, as it creates genetically identical organisms by biological means, but this particular kind of cloning has not come under ethical scrutiny and is generally treated as an entirely different kind of operation.
Therapeutic cloning is the procedure for creating stem cells genetically compatible with the patient.
The modern cloning techniques involving nuclear transfers have been successfully performed on several species: (in chronological order)
Surprisingly, an Asian scientist, embryologist Tong Dizhou, cloned a fish in 1963, 33 years before Dolly the Sheep. Apparently, he published the findings in an obscure Chinese science journal, which was never translated into English. 
However, the success rate has been very low: Dolly was born after 276 failed attempts; 70 calves have been created from 9,000 attempts and one third of them died young; Prometea took 328 attempts. With certain species such as dogs no successful clones have been created at all.
A surprising development to do with aging resulted from finds that Dolly was apparently born old; she developed breasts at age six. Aging of this type is thought to be due to telomeres, regions at the tips of chromosomes which prevent genetic threads fraying every time a cell divides. Over time telomeres get worn down until cell-division is no longer possible - this is thought to be a cause of aging. However, when researchers cloned cows they appeared to be younger than they should be. Analysis of the cow's telomeres showed they had not only been 'reset' to birth-length, but they were actually longer - suggesting these clones would live longer life spans than normal cows (but many have died young after excessive growth). Researchers think that this could eventually be developed to reverse aging in humans.
Main article: Human cloning
Human cloning is a subject of great controversy regarding its ethical and practical consequences. Many people believe that attempts to perform human reproductive cloning would be unethical, but some scientists have publicly announced their intention to do so. A number of groups have made claims that they are working on or have already produced human clones. None of these claims has been independently confirmed. Meanwhile therapeutic cloning appears to be a promising technology for combating many deadly diseases. It should be noted that natural clones occur - identical twins.
Cloning, or more precisely, the reconstruction of functional DNA from extinct species has, for decades, been a dream of some scientists. The possible implications of this were dramatized in the novel by Michael Crichton and high budget Hollywood thriller, called "Jurassic Park". In real life, one of the most anticipated targets for cloning was once the Woolly mammoth, but attempts to extract DNA from frozen mammoths have been unsuccessful.
In 2000, a cow named Bessie gave birth to a cloned Asian guar, an endangered species; this provided hope that similar techniques (using surrogate mothers of another species) might be used to clone extinct species; in anticipation of this possibility, the last bucardo, a Spanish mountain goat, was frozen immediately after it died (from illness after birth). Researchers are also considering cloning endangered species such as the giant panda, ocelot, and cheetah.
In 2002, geneticists at the Australian Museum announced that they had replicated DNA of the Thylacine (Tasmanian Tiger), extinct about 65 years previous, using polymerase chain reaction (PCR). However, on February 15 2005 the museum announced that it was stopping the project after tests showed the specimens' DNA had been too badly degraded by the (ethanol) preservative.
One of the continuing obstacles in the attempt to clone extinct species is the need for nearly perfect DNA. Furthermore, if animals were cloned from one individual, the significant problem of lack of genetic diversity would still remain in the attempt to establish a breeding population.
While the promise of cloning extinct species has been a long standing justification for the development of cloning, there are many other applications, such as cloning animals (eg. cattle and horses), which appears to offer a much faster and more efficient way of propagating desirable genes (as chosen by humans) than traditional breeding.
Another application which has recently become feasible is the cloning of pets. The company Genetic Savings and Clone was established to provide such a service, with Little Nicky being the first pet cloned by the company after the death of the original cat. The procedure is still very expensive and has little demand. However, demand could be generated from unexpected quarters, such as Hollywood movies studios, which could seek to store genetic samples of "animal actors" for the purpose of creating a clone to replace the original animal in a sequel .
Cloning has been widely explored in science-fiction.