Clone

A clone is (usually) a biological copy of another biological entity. Clones can be produced to be exact replicas with short lifespans, as the Fourth Doctor did for himself and Leela at the Bi-Al Foundation. (DW: The Invisible Enemy) Or they can be clones of individual cells which are grown up into a useable organism, as Cassandra O'Brien did for herself following the destruction of the skin taken from the front of her body, on Platform One. (DW: New Earth)

Uses of Cloning
A clone of Martha Jones was made by the Sontarans to infiltrate the United Nations Intelligence Taskforce. The clone had a mental link with the real Martha and could therefore access her memories. If the link was broken, the clone would most likely not survive. The Sontarans used many clones to try and stop the Doctor from interfering with the conversion of the Earth's atmosphere and the subsequent invasion. (DW: The Sontaran Stratagem / The Poison Sky)

Using blood samples from the Sixth Doctor, The Forge operative known as Nimrod created a multitude of clones in order to discover the means of Time Lord regeneration. All these clones were of the Doctor's sixth incarnation which suggests that Time Lord blood changes completely during regeneration. None of the clones could regenerate, they only had a 10% chance of surviving birth, and would not live very long afterwards. Nimrod has since abandoned this project. (BFA: Project: Lazarus)

Races which use Cloning
The Sontarans are a clone race, making each Sontaran almost identical to the other. However, the Sontarans have altered their biology during their long war with the Rutans to make them more formidable in battle. (PDA: The Infinity Doctors)