Seafarers, Cities and Knights, and Traders and Barbarians are all expansions for Settlers of Catan. Every expansion requires the original game to setup and play.
The various expansions can be mixed. For example, Seafarers and Cities and Knights can be combined without trouble. In some cases the rules conflict - in which case adjustments need to be made. The Traders and Barbarians game manual describes some of the possible combinations. Many more combinations are listed on Catan.com:
Combinations with "Traders & Barbarians"
The Fishermen of Catan - The Rivers of Catan
The Fishermen of Catan - The Caravans
The Fishermen of Catan - Barbarian Attack
The Fishermen of Catan - Traders & Barbarians
The Rivers of Catan - The Caravans
The Rivers of Catan - Barbarian Attack
The Rivers of Catan - Traders & Barbarians
The Caravans - Barbarian Attack
The Caravans - Traders & Barbarians
Barbarian Attack - Traders & Barbarians
Combinations with "Seafarers"
The Fishermen of Catan - Seafarers
The Rivers of Catan - Seafarers
The Caravans - Seafarers
Barbarian Attack - Seafarers
Traders & Barbarians - Seafarers
Combinations with "Cities & Knights"
Catan for 2 - Cities & Knights
The Fishermen of Catan - Cities & Knights
The Rivers of Catan - Cities & Knights
The Caravans - Cities & Knights
Barbarian Attack - Cities & Knights
Traders & Barbarians - Cities & Knights
You are not limited by these "official" suggestions. You could combine any set of Catan expansions, variants, extensions, or scenarios as long as you understand the rules well enough to resolve any conflicting rules before you start playing. That said, some combinations would be harder to resolve than others (like Cities and Knights + Barbarian Attack which have overlapping concepts with different implementations).