The 5-6 Player expansion allows for up to two extra players, which means that two new colours must be introduced. This expansion provides two new sets of wooden pieces in green and brown, both of which are very aesthetically pleasing. There are also extra resources cards and development cards which adjust the number of resources available with the extra players as well as extra terrain hexes and ports.
The biggest change which this expansion brings to the game is the extended island. With more players involved the island needs to be bigger to prevent excess crowding. There is also an added rule which allows players to trade in on-hand resources to build at the end of each persons turn. This negates the problem of being burnt by the robber easily since it takes longer between your turns and you build up more resources in that time.

This expansion requires the CATAN base game. Ideally, using the same publisher and edition of the game.