The Ood were a psychic race who offered themselves as slaves.
Biology
Oods were a humanoid species with coleoid tentacles on the lower portions of their faces. They have no vocal cord and required translation spheres in order to communicate with other species. In order to communicate with each other, the Ood share amongst themselves a low-level telepathic communication field, rated at "Basic 5". The colour of an Ood's eye change with the level of telepathic activity, glowing bright-red when there is a strong telepathic source. There are no male or female Ood, which would suggest that they are hermaphrodites. (Doctor Who: The Visual Dictionary)
The Ood are a herd race that required command to survive. The Ood offer themselves for servitude willingly, having no goals of their own except to be given orders and to serve. It is also claimed that they cannot look after themselves, and if they do not receive orders, they simply die. (DW: The Impossible Planet / The Satan Pit)
History
By the year 43K2.1, the Ood were a slave race to Humanity, performing menial tasks, and it was claimed that every Human had an Ood servant. They were generally barely even regarded as being alive, and are considered to be expendable. Despite this there is a group called the "Friends of the Ood" who are apparently lobbying for Ood freedom. (DW: The Impossible Planet)
When encountered by the Doctor and Rose Tyler, the Ood were part of the Walker Expedition. Unlike the humans or the Doctor, the Ood knew of prophecies regarding the Beast which the expedition would later discover. (TS: Tardisode 8, Tardisode 9, DW: The Impossible Planet)
The empathic nature of the Ood seemed to make them susceptible to psychic possession by the Beast, who formed the Ood on the base into its "Legion". While possessed, an Ood electrocuted a human guard by throwing its translation sphere at him. (DW: The Impossible Planet)
The Ood were defeated when Danny Bartok, the expedition member in charge of them, broadcast a telepathic flare which reduced their field to "Basic Zero", creating a "brainstorm" which caused them to collapse. However, their telepathic field began to reassert itself after a time. When Krop Tor was sucked into the black hole, the Doctor was unable to save any of the Ood on the base, who had been freed of the Beast's control, and all of them perished. (DW: The Satan Pit). All of them were given posthumous honours.
In need of heat, the Ood were looked after by the Company and then sold to citizens throughtout the galaxy (DW: Planet of the Ood).