The term cloning comes from a Greek word meaning "asexual reproduction". A clone is essentially a group of life forms that are in every way identical, but separate. In that sense, identical twins are also clones, because of the way in which they form. Clones are formed by division which also known as "vegetative reproduction".
Cloning comes in three forms that are vastly different in how they are performed. Embryo cloning, also call "artificial twinning", is the first form is a medical technique that copies the process that forms triplets or twins, this has been tested on many animals and has been proved successful. However it has scarcely been performed on humans, due to strong controversy. The second form of cloning is Adult DNA cloning. This process, also known as cell nuclear replacement, involves creating a duplicate of an existing adult animal. This was the process which produced "Dolly", involves taking the DNA from and adult animal and placing it where the DNA of a new embryo would be. Thus, producing and exact duplicate of that animal. This process may have the ability to create a identical twin of a living human, but has not been attempted on humans. Therapeutic cloning, the third process, is similar to the previous method however it removes the embryo's stem cells which is perceived to able to form parts of human tissue or whole organs for the purpose of transplanting into the DNA donor, with the sick organs.