The Daleks were born without any ability to feel compassion or pity and were motivated only by hate, fear and an implacable belief that they were the superior creatures in the Universe.
They have battled the Doctor many times throughout the centuries, with confrontations on Earth, Skaro and planets right across the universe from Exxilon to Spiridon. Powerful, pitiless and with a genius for warfare they have proven fearsome opponents for the Doctor but he has always managed to protect humanity from their attacks. They return in the new series... let's hope the Doctor can maintain his record against his most enduring enemies.
The Smilers are androids, mounted in booths all across the space faring version of Britain that left Earth in the 29th Century. They let people know what's good and what's bad. The terrified population keep them clean. Their heads can rotate to show three faces: happy; warning or very dangerous indeed.
And if things get very dangerous, the Smilers can leave their booths and move around. They're actually androids, an extension of the 'starship's' security system.
There are also the Winders, half Smiler and half human, who've taken an oath to serve the system. And all together, these unhappy people and automatons are doing a terrible thing, which only the Doctor and Amy can save them from.
Prisoner Zero is one of an interdimensional multi-form species, who, having created a psychic link to a living but dormant creature, can then copy the appearance of that creature or of anything they dream about. Coma patients provide the perfect live feed: Prisoner Zero hides in the dreams of the living dead.
The being's natural form is gelatinous, a snakelike mass of glowing tendrils with a mouth full of sharp teeth. It's capable of creating a perception filter to stop itself being seen, or to hide anything it wants to hide.
Multi-forms can live for millennia. This one has escaped from being the prisoner of the Atraxi. Prisoner Zero is so dangerous that, to stop it escaping again, they're willing to destroy the world.
Possessing a natural and unique defence mechanism, the Angels are quantum locked. This means that they can only move when no other living creature, including their own kind, is looking at them. As soon as they are observed, they instantly turn to stone and cannot be killed.
The Angels have the ability to send other beings into the past. This allows the Angels to feed on the potential time energy of what would have been the rest of their victim's lives.
In The Time of Angels the Doctor and Amy along with River Song went hunting for a solitary Angel but discovered an army of them waiting in the Maze of the Dead! Although the Lonely Assassins vastly outnumbered their prey the Doctor was able to outwit them using gravity and the army was defeated. But there's no telling where the Weeping Angels might strike next... the Doctor must remain on his guard against these beautiful but merciless killers.
There really are vampires: the Doctor's met them. But the vampires of Venice are something else: sea-dwelling refugees from the destroyed world of Saturnyne.
The true form of such creatures is a dark, leathery beast, something like a shark. They hide that body beneath a perception filter that manipulates the brainwaves of whoever's looking at them. Which means that sometimes you won't see them in mirrors. But a human's sense of self preservation means they're always aware of the fangs. In their natural form these vicious predators live underwater.
To continue their species the disguised survivors of Saturnyne drank from their female human victims until all the liquid had been removed from them. Then they replaced that liquid with their own blood. Gradually, the alien blood changed the victim into one of the aliens, ready to mate with the male aliens, hiding and feeding in the Venetian canals.