However, you should master the basic game before moving on - the expansions are for people who really like the game and want it to last (often much) longer and provide deeper gameplay.
Catan has numerous expansions - one enables 5-6 players, and others include Seafarers, Cities & Knights, and Traders & Barbarians. Most of the advice I've given still largely holds, though many subtleties creep in and interesting additional mechanisms are brought into play. However, you should master the basic game before moving on - the expansions are for people who really like the game and want it to last (often much) longer and provide deeper gameplay. There is even a Star Trek Catan if you'd rather trade dilithium than sheep...