The word clone comes from "klon," the Greek word for "twig" or "branch." In biology, it refers to individual organisms or cells that are genetically identical.
The two techniques for cloning mammals are embryonic cell cloning and somatic cell cloning. In embryonic cell cloning, viable blastomeres are selected from a fertilized egg that has started dividing into cells numbering between 16 and 32. These cells are implanted into an unfertilized egg from which the nuclear has been removed, and then the egg is implanted in the womb of a surrogate mother to gestate until birth. This enables the production of many genetically identical individuals, but it does not result in exact genetic copes of the parent.
In somatic cell cloning, the nuclear of a cell from skin or other tissue is fused with an unfertilized egg from which the nuclear has been removed, and then the egg is implanted in the womb of a surrogate mother to gestate until birth. This technique produces nearly identical genetic copes of the parent, but due to blending of intracellular mitochondrial DNA and other factors, such as differences in the rearing environment, the cloned individuals will not necessarily develop in the same manner.
There is also a theory that telomeres, which play a role in cellular aging, shorten in cloned individuals.