The second serial, The Daleks, knocked the ball out of the park by creating a fearsome foe that became more recognizable than the Doctor himself. (His name isn't actually Doctor Who, by the way, just the Doctor, but "Doctor? Doctor who?" has been a running gag in the series since the very first episode.)
The Daleks' appearance is more comical than fearsome; they look like human-sized, rolling pepper shakers with one long eyestalk and two arms that resemble a plunger and an eggbeater. They're like sinister versions of R2D2 -- who, of course, was created more than a decade later. The most unnerving thing about them is their voices, harshly distorted staccato screams that make them sound like they're in a constant state of hysteria. And what they scream more than anything else is "Exterminate!"
One of the ground rules that BBC Head of Drama Sydney Newman dictated when he came up with the basic concept for Doctor Who (and left it to others to flesh out) was that there were to be no robots or "bug-eyed monsters" on the show. Created by writer Terry Nation and designer Raymond Cusick, the Daleks appeared to be both these things at once. But the Daleks aren't robots at all; the machine-like appearance is just a hard shell with a chewy center. Inside are shrunken, hideously deformed mutants with tentacles.
Daleks were once a humanoid race called the Kaleds that looked identical to humans. They lived on the planet Skaro with another human-looking race called the Thals, with whom they engaged in a long war, leaving their planet an irradiated wasteland. The scientist Davros deliberately mutated his fellow Kaleds into the Daleks, encasing them in mechanical armored shells and removing their ability to feel compassion, pity or remorse, leaving them with nothing but hate and a thirst for the conquest and extermination of all other species.
The Daleks in the upcoming Christmas special 'The Time of the Doctor'
The Daleks proved hugely popular with viewers, helping to make the show a bigger hit than the BBC could have anticipated, and they appeared again and again over the decades, though never often enough to make their threat seem routine. (Terry Nation having a share of the intellectual property rights may also have been a factor in using them sparingly.) Whenever the Daleks were involved, you knew the stakes had been raised.
The Daleks appeared in only about 19 of the 155 serials of the original series, including such minor appearances as an empty shell in a space museum or a flashback to an earlier adventure in an episode in which they weren't otherwise involved. They would sometimes go as much as five years without appearing at all. They're a bit more ubiquitous in the relaunched series, showing up in 21 out of 104 episodes to date, although here again that includes cameo appearances. They were the focus of two Peter Cushing movies in the 1960s (Dr. Who and the Daleks and its sequel Daleks -- Invasion Earth 2150 AD) loosely adapted from the early serials The Daleks and The Dalek Invasion of Earth, although Doctor Who fans tend to pooh-pooh the movies for the many liberties they take. (The Cushing character is a normal earthling who happened to create a time machine.)
There was a short-lived comic strip in the UK in the mid-'60s, The Daleks, that was entirely separate from Doctor Who's own strip. They inspired a novelty song in 1964 called "I'm Gonna Spend My Christmas with a Dalek" by the Go-Go's (not the 1980s girl group) and have been name-checked in songs by The Clash and The Creatures. They've been the subject of comedy routines by Eddie Izzard and Spike Milligan and have been spoofed on South Park and Community. They even popped up in the movie Looney Tunes: Back in Action.
One thing that's remarkable about the Daleks is how little they've changed since their first appearance. They've gone from silver to gold to candy-colored and back again, and they've been given a little more variety in their arm implements, but their iconic design has remained remarkably the same as the 1963 original, as has their agitated robotic voice.
'Asylum of the Daleks,' 2012
The Doctor, on the other hand, has changed a lot. The series has managed to last as long as it has in part due to a particularly clever workaround introduced early on: From time to time the Doctor regenerates, switching to an entirely new body with a somewhat different personality, which allows the show to replace its lead actor whenever necessary without missing a beat. In fact, that's due to happen in the special that airs on Christmas Day, The Time of the Doctor, when Eleventh Doctor Matt Smith is to be replaced by Twelfth Doctor Peter Capaldi.
The Daleks may look more or less the same, but they've become considerably more formidable over the years. For a long time there was a running gag about Daleks being unable to climb stairs, which was eventually disproved when it was shown that they could hover. Sometimes it even seemed as if they'd been eradicated, but like any good bad guys, they got better. With their nearly impenetrable "dalekanium" armor, superior firepower, time travel and weapons of mass destruction, not to mention their ornery temperament, they're one of the most fearsome threats in the universe.
What the Daleks want can be simple (what part of "EXTERMINATE" don't you understand?) or bizarrely complex, such as destroying the Earth's magnetic core and flying the planet away. Sometimes they're just chasing the Doctor through time, sometimes engineering political alliances to bring about a future in which they rule all. But they're motivated entirely, indefatigably, by their own sense of absolute superiority to all other beings, which they either exterminate or enslave as long as they prove useful, to be exterminated later.
When the Doctor first encountered them in The Daleks, the pacifist, human-looking Thals and the Daleks had an uneasy coexistence on their home planet of Skaro until the Daleks tried to wipe out their old enemy by flooding the planet with the radiation on which they themselves thrive. The Daleks' obsession with their own innate superiority coupled with their need to adapt to ensure the survival of the species has often led to various factions of mutated Daleks trying to destroy each other as inferior abominations.
Several times it's appeared as if the entire race of Daleks has been destroyed, but time travelers are notoriously hard to eradicate and inevitably show up in another era. In some futures they've built mighty galactic empires that have lasted for centuries, and many of the Doctor's adventures have either hinged on destroying those empires or preventing those futures from happening in the first place. Once the 1970s Doctor, played by Tom Baker, was sent back in time to exterminate the Daleks at the moment of their creation, but he couldn't bring himself to do it. The Daleks showed up in 1930s New York trying to breed a new race of human/Dalek hybrids. They've posed as robot helpers serving tea in Winston Churchill's war room while working to rebuild their race, and they've attempted to distill a "Dalek factor" and insert it throughout human history. Once they even stole a number of planets, absorbing their energy to create a "reality bomb" that would destroy reality itself.
'The Dalek Invasion of Earth,' 1964
A 2006 episode, Doomsday, brought the Daleks into conflict for the first time with another of the Doctor's oldest enemies, the Cybermen (a race of forcibly converted cyborgs, much like the Borg on Star Trek). By far the best part was the trash talk between the two conquering armies of flesh-and-metal hybrids (see video below). When asked by Cybermen's Cyber Leader, "We have five million Cybermen. How many are you?" the Daleks replied with supreme confidence, "Four!" They elaborated, "We would destroy the Cybermen with one Dalek! You are superior in only one respect. You are better at dying." Besides the basic badassery of four Daleks being a match for millions of Cybermen, their number may have been a sly reference to the fact that only four Dalek props were constructed for their first appearance in 1963.
The BBC seems to have expended all its Whovian energy on the 50th anniversary special and the upcoming Christmas special, to the exclusion of any fitting tribute to the Daleks' own golden anniversary. The deadly pepperpots at least appeared in The Day of the Doctor, not as the main adversary but as part of the background involving the Time War between the Daleks and the Doctor's people, the Time Lords. From advance photos, it looks like the Daleks show up in the Yuletide episode as well, though they'll have to share the spotlight once again with the Cybermen, who should really be exterminated for the indignity. Even if the Daleks aren't given their due on their own golden anniversary, such ephemeral markers are meaningless when you can travel through time at will. A mere five decades is nothing for a race destined to dominate the universe for all eternity.