A module is another name for a published pre-made adventure scenario. Typically, a module will provide the following:
- at least one and possibly several plot hooks to engage the party
- detailed maps of all significant adventuring areas
- descriptions of adversaries in pre-planned encounters
- guides to the behavior of adversaries in pre-planned encounters
- an intended storyline for the characters to follow
- a guideline for the overall party strength required to successfully complete the adventure
The most praiseworthy feature of such modules is that they allow referees with little time for detailed construction of challenges to run interesting and engaging gaming sessions. Their greatest flaw is that they often tend heavily toward railroading.
Possibly the most famous module is "B2: The Keep on the Borderlands" which was packaged with the Dungeons & Dragons Basic Set.