Settlers of Catan: Expansion vs. Extension???

[starrzone]

My brothers are hard to buy for, but they love playing Settlers of Catan; I was looking at Boxing Day Sales (I know, I know!) and found there are 2 sorts of "add-ons"; expansions and extensions. Can anyone explain to me in layman's terms the difference between the two??

 

[Meriweather]

The expansions are the add-ons that change how the game is played such as Seafarers or Cities & Knights.

The extensions are the 5-6 player add-ons. The base game and each expansion have an associated 5-6 player extension.

 

[Skippy918]

An expansion adds different components to a game, while an extension adds additional players. Some games combine both, but on others it’s separate.
Here’s one I found for Catan.

http://www.boardgamebarrister.com/guide-to-settlers-of-catan-expansions/