Poosh was the name of either a moon, or the planet around which this moon orbited. Poosh was known for its fantastic swimming pools and, alongside Paris, was one of the two places to which the Doctor wished to retire. (PROSE: The Vampire of Paris)
At some point, the moon disappeared. Efforts to find the moon were unsuccessful. Dee Dee Blasco had become a research assistant to Professor Hobbes on the strength of a paper she had written about Poosh. The timing and circumstances of Poosh's disappearance were not revealed to the Doctor. (TV: Midnight)
The Tenth Doctor later discovered that the Daleks had stolen this planetary body out of time and space and hidden it in their time pocket in the Medusa Cascade along with twenty-six others in order to power Davros' reality bomb. (TV: The Stolen Earth) When the planets that had been stolen were being returned, this was one of the ones returned by Donna Noble. (TV: Journey's End)
Behind the scenes
The script for Midnight is slightly vague about whether "Poosh" names the moon or the planet around which the moon orbits. Because the phrase "the lost moon of Poosh" is employed, it is possible to think of Poosh as the planet, and the moon being unnamed. However, the Doctor proposes a toast to "Poosh" and Dee Dee accepts the toast without correction. In the context of the conversation - during which the Doctor suggests that Dee Dee's contribution to science might be finding the lost moon - Poosh is more likely the moon's name, rather than the planet's. This seems to have been confirmed by the holographic image of the planetary body seen in The Stolen Earth. However, the script to this later story muddies the waters further by calling Poosh "one of the 27 planets" stolen by the Daleks. It could also be said that the word 'planet' was used loosely here, and is synonymous with the term 'planetary body' under which a moon would fall. The mentioning of there being "moons of Poosh" in The Forgotten Army would suggest that Poosh is indeed a planet rather than a moon.