Most adventure modules are intended for single players, but some are multiplayer by intent and some are written to be played with a DM. These are created using the game's toolset, and mostly are completely unrelated to the original game. The most popular mods are adventure modules. Mods can be divided into ones that are intended primarily for players and those that are of most use to builders.
Some are created by a single person, while others involve large modding teams. They can be small or large, simple or complex. A mod is any piece of content that adds to or alters the original game.