Dalek

The Daleks are a breed of genetically altered Kaleds from the planet Skaro. The Daleks were created by a megolomanial Kaled war scientist called Davros under orders from the Kaled high council. The project was designed to build a weapon capable of defeating the Kaleds ancient enemy, the Thals; Davros betrayed the Kaled council and instead designed his own supreme being, capable of surviving the nuclear wasteland that Skaro had become. Davros removed from his creation all emotions other than hatred and contempt. The Dalek alien is a genius programmed from birth to regard all other species as inferior, a lack of foresight on the part of their creator as the Daleks eventually killed him (although he returned from death later on.) Before their fall at the hands of the Time Lords the Dalek race split into two factions, the renegade Daleks, under the command of the Supreme Dalek, and the Imperial Daleks, under the command of the Dalek emperor (the resurrected Davros.)

Daleks have force shields that make them very difficult to kill. They also have very powerful weaponry. Famously they cannot climb stairs, but they can hover (or, as they put it, elevate) to solve this problem. Daleks also have the ability to time travel, both using their own time machine and by performing a temporal shift (this ability may be limited to the Cult of Skaro). At the height of their power, 4 Daleks could conquer the World (this may have been a taunt).

The Daleks wish to exterminate the species they consider inferior (i.e. all of them). They shout 'Ex-ter-min-ate!' before fighting and/or killing; they rarely use the word 'kill', preferring 'exterminate'. Talking about their execution of the Master, an evil Time Lord, he says that he was 'exterminated' by the Daleks, and Captain Jack refers to 'death by extermination'.

The Time War: Spoilers Ahead!!
The Daleks opposed the Time Lords in the Last Great Time War. Many worlds were destroyed in this war, including Skaro and both species were nearly wiped out. The Doctor thought he was the only survivor, until he came across a Dalek that had been captured and tortured by Humans. The Dalek Emperor also survived and created an army of Daleks from the bodies of dead humans. These Daleks, hating their own existence (coming from an inferior species).

On the verge of conquering Earth, they were destroyed by Rose Tyler, the Doctor's human companion, who had absorbed the power of the Time Vortex. The Cult of Skaro, 4 Daleks whose purpose was to imagine and think like their enemy (regular Daleks have no imagination), survived by hiding in the Void. Their leader, Dalek Sek, was imprisoned by the other 3 when, after turning himself into a Human-Dalek hybrid, he said that Daleks were not supreme and planned to continue the species by creating hybrids using the bodies of humans whose minds had been destroyed. He was later accidentally exterminated when he tried to save the Doctor, who Dalek Sec claimed was the key to saving the Daleks. His experiment was interrupted by the remaining members (breaking the law that 'Daleks must obey'), and the results were humans with the minds of Daleks. However the Doctor got in the way of the gamma beam, mixing in Time Lord DNA, so the Dalek Humans had curiosity and some free will, and destroyed 2 more members of the Cult of Skaro. The final member, Dalek Caan, escaped from the Doctor (who was offering to help him) using an emergency temporal shift, and the Doctor said that they would meet again. When they did, Caan had travelled to and from the Time War and brought back Davros, the original creator of the Daleks. He created an army of real, true Daleks and created a time bomb out of random planets. The Doctor eventually destroyed the entire army and left Davros in an exploding ship.