Aircraft Carriers were warships that had a runway for planes to land and takeoff on. These ships did not have any traditional warship equipment on them (gun artillery, torpedoes, etc.) and instead only had a supply of airplanes and lifeboats. They had on armor on them and were vulnerable to hits from other bigger warships, as one good hit from another warship onto the runway could suspend the aircraft carrier from performing out any action for a good while until it was repaired. They revolutionized naval warfare as being ships with great power, able to send a portable aerial runway into a naval battle without having to be close to land. Aircraft on aircraft carriers were much smaller than Land-Based aircraft. Some types of armament that was carried on said airplanes were conventional bombs, torpedoes, rockets, depth charges and rarely atomic bombs (after World War II). Aircraft Carriers were a great counter to the big Battleships that used their big belt armor to protect themselves. Since most aircraft allowed for the bombs to be dropped from above, the airplane's bombs now entered in via the deck, where it was less armored, thus being deadly.
You can land on most Carriers.