Cloning refers to the duplication of genetic material from another living organism, such as a plant or animal.
This can occur in nature, as happens with the reproduction of bacteria and algae, or plants such as strawberries.
However, in recent years it has also become possible to manufacture clones through scientific intervention.
This engineered cloning has sparked great debate, but in fact there are three very different types of cloning, offering three very different results.
Embryo cloning is similar to the process that produces identical twins.
Cells are removed from a fertilised embryo and encouraged to develop into duplicate embryos with identical DNA.
This method is widely used in animal breeding and, since no embryos are destroyed and human experimentation is rare, has aroused little debate.
In contrast the two other forms of cloning - reproductive and therapeutic - are both highly controversial due to their methods and desired results.
Reproductive cloning is contentious as it seeks to create a living duplicate of an existing animal.
DNA is removed from an egg and replaced with DNA taken from an adult animal, making a genetic twin.
Then the fertilised egg, now called a pre-embryo, is implanted in a womb and allowed to develop into a new animal.
Until the birth of Dolly the sheep in 1996, there was doubt this would work in mammals, but now some scientists say they are trying to clone a human.
Therapeutic cloning follows the same initial stages as reproductive cloning, but once the copy pre-embryo has been created its stem cells are removed.
Stem cells can grow into any type of cell, so researchers hope to harvest them and use them to replace ones that have become worn out or damaged.
This could represent a powerful and versatile tool for fighting disease.
But the pre-embryo, which some consider to already be a human with a right to life, is destroyed in the process.
Supporters of cloning say it could offer numerous benefits in the future.
Therapeutic cloning might create perfect-match organs for transplant or cells to fight degenerative human conditions like Alzheimer's disease.
Reproductive cloning could give infertile couples the chance of a baby, or preserve an endangered species.
Cloning could be an ideal way to mass-produce elite farm animals with particular valuable qualities, such as dairy cows with a high milk yield.
For opponents any rewards cloning may offer are overshadowed by moral unease and fears scientists are "playing God".
Cloning for stem cells destroys the pre-embryos used, while reproductive cloning risks the lives of both mother and clone, with most attempts failing.
Critics also say clones may suffer untold physical and mental problems.
Many experts believe the debate for and against cloning will be the battle that will define biotechnological practices throughout the 21st Century.