Every game has details. Role playing games have details. It is easy to overwhelm players with information. Even
people who have played the game for years do not necessarily have the patience
to pour through the volumes of information on the Forgotten Realms. Even new settings, like Golarion,
 have several tomes that detail the economy of the author's fantasy 
world. It is all interesting, but it is a lot to ask of people in order 
to play a game.
A
 lot of game masters that run a game create a large portion of their 
world before it starts, and help their players into the world; they 
usually refer to their setting as being home-brewed. A 
home-brewed setting can make for a very rich game if the game master put
 a lot of thought into it and the players are interested and respectful 
of the game master's work. It could also become a time for the game 
master to give lectures on make believe anthropology and history, with 
players begrudgingly stacking dice between "epic" encounters.
When
 4th Edition Dungeons and Dragons game came out, the Dungeon Master's 
Guide described a setting it called "Points of Light". This was a 
setting wherein the world was assumed to be a dangerous, unexplored vast
 wilderness, which was darkness, punctuated with small pockets of 
civilization, which were the points of light. Player's characters were 
the brave, desperate, and adventurous men and women who carried the 
torch into the night. This "Points of Light" setting was a description 
of some of the oldest Dungeons & Dragons game's settings.
This
 concept of a few disparate, disconnected pockets of safety in the 
vastness of ruins and dungeons creates a lot of opportunity for ad hoc
 session planning. Right now, in "When the Trees are Teeth", the 
player's characters are in the town of Drafton. I have town detailed, 
and the surrounding wilderness, and a large dungeon nearby to explore. I
 have my next handful of sessions planned, but I do not have any grand 
agenda as to where the party will end up.
I
 can make assumptions about the game's setting by drawing on archetypes 
in the fantasy genre. For example, I know there is a wizard's college 
somewhere, a thieves guild, a king, foppish nobles, a bard school, and 
dragons. When I am idle and bored, I may think about who the king is, 
and how he gets along with the bard's college, but it is not relevant to
 the game right now. In role playing games, a thing does not exist until
 that thing becomes relevant to the game.
More interesting about the ad hoc setting
 is that it provides freedom to give creative license to the players. If
 a player wants to suggest that he comes from a land similar to the 
mythic far east, then it fits! An ad hoc setting creates a rich world by player consensus. 







