Short answers, pulled from the story.
The BBC Television Service broadcast the first episode of Doctor Who at 17:16:20 GMT on the 23rd of November 1963. This transmission occurred just eighty seconds after its scheduled time due to news coverage regarding the assassination of John F. Kennedy the previous day.
A Time Lord can only regenerate twelve times, creating thirteen incarnations total according to the rules established in The Deadly Assassin. The episode The Time of the Doctor depicted the Eleventh Doctor acquiring a new cycle of regenerations when facing the end of his original set.
The Daleks first appeared in the show's second serial in 1963, created by writer Terry Nation and designer Raymond Cusick. These Kaleds from Skaro mutated by scientist Davros wear mechanical armour shells housing octopus-like creatures with large brains while Cybermen originated on Earth's twin planet Mondas before implanting artificial parts into their bodies to become coldly logical cyborgs.
Doctor Who ran for twenty-six seasons on BBC One from the 23rd of November 1963 until the 6th of December 1989. Channel controller Michael Grade cancelled the upcoming twenty-third season in 1985, forcing an eighteen-month hiatus before Russell T Davies and Julie Gardner revived the programme in 2005 with BBC Wales producing in Cardiff.
Ninety-seven of 253 episodes produced during the first six years remain missing from the BBC archives. Most notably, seventy-nine episodes are lost from seasons three, four, and five because large amounts of older material stored in the BBC's video tape and film libraries were destroyed or wiped between 1967 and 1978.