The Carrier is one of the final naval units and has room to carry 3 Fighters. They are very weak unsupported, so they should only be used with escorts or against a civilization with a small or nonexistent navy.
The aircraft carrier first came into widespread use during World War II. Carriers act as floating airfields, carrying fighters far from friendly territory to be launched against enemy targets. This allows an air strike against targets on both land and sea that would otherwise be far out of range.
The range of the aircraft launched from carriers forever changed the methodology of naval battles. Opposing fleets could now engage in battle without either fleet ever entering the other's visual range, ending the dominance of the battleship as the strongest vessel on the sea.
The Carrier appears to be based upon the Nimitz-class aircraft carrier.