The Pod people was a nickname given to an alien species featured in the 1955 novel the Body Snatchers by Jack Finney, the 1956 film Invasion of The Body Snatchers, the 1978 remake and the 1993 film Body Snatchers. There is of course also the up and coming 2007 film, Invasion.
The pod people are a race of nomadic extraterrestrial parasites originating from a dying planet. Realizing that it was only a matter of time before the planet's resources would be completely depleted, the pods somehow evolved the ability to defy gravity and leave their planet's atmosphere in the search of a new world to colonize.
For millions of years, the pods floated in space like spores, propelled by the solar winds, some occasionally landing on inhabited planets. There, they'd replace the dominant species by spawning emotionless replicas, and use up all of the planet's resources, only to then leave in search of yet a new world to colonize. This apparently was the fate of the civilizations that once inhabited Mars and the Moon.