Summary: There are all kinds of aliens in the universe.
“Aliens come in all shapes and sizes, Rose,” the Doctor explained. “Most of them look nothing like you’d expect.”
“So no little green men from Mars then?”
“Oh, they exist, but they’re not from Mars. There are little red men too, and little orange men, except they’re not really men, not like human men. People don’t have to have two arms and two legs, or hair on their heads. They might have tentacles, or wings, or feathers. Look around you.” He gestured at the alien market place. “Aliens everywhere.”