Clone

A clone was (usually) a biological copy of another biological entity. Clones could 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 could be clones of individual cells which were grown 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 access her memories. If the link was broken, the clone would most likely not survive. The Sontarans used many clones to try to 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 changed completely during regeneration. None of the clones could regenerate. They only had a one-tenth chance of surviving birth, and would not live very long afterwards. Nimrod since abandoned this project. This was most probably because of Time Lord physiology, suggesting traditional cloning techniques would not work. It may also have had something to do with the Rassilon Imprimatur. (BFA: Project: Lazarus)

Clones of the Three Who Rule were produced. (NA: Blood Harvest)

Clones of Galifrey were created during the the War against the Enemy and came to be known as Nine Gallifreys. (EDA: The Shadows of Avalon)

Santa Claus used clones of himself to deliver presents on Christmas Eve. (ST: The Man Who (Nearly) Killed Christmas)

Races which use cloning
The Sontarans were a clone race, making each Sontaran almost identical to the other. However, the Sontarans had altered their biology during their long war with the Rutans to make them more formidable in battle. The fact that the Sontarans erre a clone race however, most probably remainrf unchanged. No other evidence indicates otherwise. (PDA: The Infinity Doctors) Mr Smith suggested that the Graske, who looked alike, may have been a clone species like the Sontarans. (SJAF: Sarah Jane's Alien Files episode 1)