From Dungeons and Dragons Wiki
Generally, there are three types of campaign settings:
- A campaign setting specifically designed to be played in D&D: for example, Forgotten Realms or Eberron.
- A campaign setting vaguely defined by its main features: for example, one can say that the adventure takes place in a "Medieval fantasy setting", implying that there will be dragons and swords, but no space ships or guns. "Generic D&D" belongs here, because it claims to use Greyhawk as a basic setting since 3e, but almost never uses advanced world features like the timeline.
- A campaign setting adopted for D&D play: for example, some campaign settings were initially designed for different systems, but were migrated to d20 system. Campaign settings made to support gaming for certain books, legends or periods in real history also belong here.
There are numerous campaign settings available both in print and online. In addition to published campaign settings available for purchase, many dungeon masters create their own.