The jokes were an addition to TF2 added on August 10th, 2012. They could be created by equipping the Damaged Capacitor in a miscellaneous slot (this functionality was added with the update) as well as the High-Five Taunt in the action slot, and taunting, producing an output 'joke' in the ingame chat. The jokes were combined by the community into various codes of different types. These codes were then deciphered in their various forms and were related by the PGP Word List.
Medic says: Did you hear the story from 1252 where Destruction PDA, Buzz Killer, Something Special for Someone Special, Übersaw, Cowardly Mound, Crate, Buzz Killer, Frying Pan, and the Administrator walked into a bar?
This joke relates to the 'medic' sequence of joke data and '1252' relates to the offset the data should be placed in. The items listed were assigned hexadecimal values and arranged in the order given in the joke, thus forming a bit of hexadecimal code.
Jokes could be submitted to xPaw's data collection webpage, viewable here.
The data is further collated on XKeeper's webpage, here.