What is a Dalek?
Daleks are (possibly the most familiar) antagonists in the British sci-fi series, Doctor Who.
They are Kaled mutants operating within individual mobile weapons platforms constructed from dalekanium (super-strong alloys with polycarbides). The Kaleds originated on the planet Skaro as great philosophers and scientist who were waged war upon by their planetary companions, the war-like Thals. Generations of bitter and increasingly vicious warfare ensued between Thals and the Kaleds on Skaro (the Thousand Year War).
The Thals succeeded in quashing the Kaleds and settled down to a more peaceful existence, but this did not last for more than a few centuries before hostilities returned. While technologically well-matched, the Kaleds focused on surviving the wrath of the Thals while the Thals focused on utterly destroying the Kaleds, resulting in a tactical stalemate as the Thals designed horrifyingly destructive weapons, but were unable to overcome the Kaled's defences- neither side was capable of achieving victory. Both races ultimately rendered their planet a wasteland, surveying all they had wrought from the protection of their domed cities.
The environment had been poisoned with high levels of radiation (to which the Thals were greatly more resistant). The Kaleds suffered progressive mutations until their withered bodies struggled to sustain them. They developed self-contained life-support and transportation units to move around in according to the design of the crippled and twisted chief Kaled scientist, Davros, whose demented machinations (including the further corruption of the Kaled genetic constitution) lead him to become the progenitor of the Daleks as they are now known- combining his genetically-engineered mutants with weaponised 'travel machines' based upon his own mobility device.
Davros eventually betrayed the Kaleds by providing information that would allow the Thal to penetrate Kaled defences. When the Thals destroyed a Kaled city, Davros used this new and horrifying threat as reason to unleash his Daleks- the ultimate weapons. Despite a few obstructions and hiccups caused by interference from early incarnations of The Doctor, the Daleks rose to found a vast inter-galactic empire, ultimately waging war upon the Timelords themselves.
The Dalek catchphrase is "EXTERMINATE!" Their voices (loud, modulated machine-like monotones) are far from pleasant, and help to enhance their rather unfriendly countenance, especially when barking their catchphrase as a ice-breaker when meeting (read: conquering) new species..
Outwardly, Daleks are mobile weapons platforms, standing as tall as (or taller than) a human, with a domed 'head' from which a rod-like eye-stalk projects terminating in a spherical 'eyeball' containing their visual sensory equipment, and usually a light either side that illuminates when the Dalek 'speaks'. The front mid-section of the Dalek hosts (usually) two appendages; conventionally a dis-integrator ray (a formidable energy weapon which produces a distinctive sound when fired) and a telescopic, multi-role, plunger-like manipulator device. There are some historical variations upon the design, including Daleks with clamps/grippers or tools in place of their manipulator appendage, heavy-weapons Daleks with large-bore weapons and heavier armour, and command/coordinator Daleks with large, spherical modules in place of the dome.
The lower portion of the Dalek chassis, the 'skirt', is embedded with a number of metallic spheres which are presumed to form part of a sensor array, though these have been seen to detach in one Dalek self-destruct sequence, hovering in formation around the Dalek and generating a bubble field around the Dalek prior to its disintegration. Daleks can generate a localised shield which protects them almost completely from all kinetic and most energy weapons, though their armour is impenetrable to most conventional weapons anyway.
Daleks have a hierarchy whose ranks are denoted by various colour schemes and small size/design variations. Despite their unshakeable belief in their own superiority as the Universe's master race, civil wars, racial differentiation (through further genetic manipulation), and even theological differences have led to various splits in their factions; Daleks, it would appear, are not above exterminating each other once in a while. In fact, it is not entirely uncommon for Daleks to exterminate inferiors who fail to meet performance targets.
Initially thought to achieve locomotion via a spherical, uni-directional bearing upon which the entire chassis is mounted and requiring metallic paths and/or distributed energy fields to provide power, it has since been revealed that Daleks now actually hover just above the ground, and are able to effectively fly when required. They have also been seen emerging from complete submersion in water and operating in space without difficulty.
The creature inside the machine shares little in appearance with its humanoid Kaled ancestors. Whereas Davros was roughly humanoid, though horribly disfigured and partially integrated with his own mobility system, Daleks seen outside of their protective war machines are not even remotely humanoid. Their current form is reminiscent of an octopus, with a number of tentacle-like appendages, vestigial facial features, an exposed brain, and a single large eye. They are not to be underestimated in this vulnerable condition, however, as they are as hateful and violent even without their weapons, and possess the strength to kill a target via strangulation. Earlier depictions of Kaled mutants have included green blobs with tentacles, and even crab-like mutations with claws.
Their most reviled enemy is the renegade Timelord known as The Doctor, who has repeatedly thwarted their evil, xenophobic plans. They covet the technology of the Timelords, and strive to similarly master space and time, if only to conquer it utterly; they have enjoyed some success with their own endeavours, but nothing so sophisticated or advanced as Timelord Tardis technology. The Doctor's Tardis, an obsolete model that was retired from service long before The Doctor stole it, is possibly the last known in the Universe (just as The Doctor is possibly the last living Timelord) as a result of the Time Wars which resulted in near-absolute destruction of both Dalek and Timelord races thanks to a final solution employed by The Doctor himself ("near", as the odd Timelord or lost Dalek warship still crawls out of the woodwork to stir things up once in a while); for this reason, obtaining it would be a great prize for any remaining Daleks, and could spell the beginning of a new Dalek age... and a brutal end for all life everywhere, everywhen!
Terry Nation created the concept of a Dalek, telling the press that the 'Dalek' name originated from spines of encyclopaedias, and 'simply rolled off the typewriter'.