Short answers, pulled from the story.
A Dungeons and Dragons adventure module is a pre-packaged book or box set used exclusively by the Dungeon Master to manage player knowledge and activities within a specific scenario. It typically contains background information, maps, location descriptions, creature statistics, player visual aids, and suggested rules for evaluating events.
The first published Dungeons and Dragons scenario was "Temple of the Frog," included in the 1975 Blackmoor supplement. The first stand-alone adventure module was Palace of the Vampire Queen, published in 1976 by Wee Warriors, with TSR distributing the first three printings.
TSR began assigning alphanumeric codes to modules in 1978, starting with G1 for Steading of the Hill Giant Chief. The coding system continued until the end of 1993, when it was dropped, and was later revived in 2008 with H1 Keep on the Shadowfell for the 4th edition.
The exact differences among adventure, module, scenario, and accessory are hard to precisely define in Dungeons and Dragons terminology, as all terms have been used in different ways. "Module" stems from the earliest TSR publications, while Wizards of the Coast currently uses "adventure" as the standard term.
All early Dungeons and Dragons modules are now out of print, and many are highly sought-out collector items, particularly the earliest printings. Some have been reprinted in revised form, but the original editions, especially those from the 1970s, carry particular collector interest.
Beyond maps, adventure modules typically included background plot information, vignettes of interesting locations, site inventories, creature descriptions and statistics, player visual aids, and suggested rules for evaluating events and likely player actions. Some also included numerous illustrations, and formats ranged from single booklets in cardboard covers to top-spiralbound notebooks for lines like Dark Sun.