A large, flat-topped boaty-thing full of seamen (Not semen!) - and latterly the RAF, since some bright spark at the MoD decided it was a good idea to fcuk up tried and tested methods and make things confusing.
Aircraft carriers are so called because they carry aircraft. They're also the numero uno target for any potential adversary in a naval battle scenario, and such prized assets are protected accordingly - in theory. Considered by submarine types as a 'stonking great target and first to get a torpedo come a war'.
Having a carrier or two in one's fleet admits one to a pretty exclusive club in naval circles.
Considered 'not much kop' up until the 1930's, the absolute beasting the Italians got at Taranto in 1940 from the Fleet Air Arm AND the one the Spams got from the Imperial Japanese Navy at Pearl Harbour in 1941 catapulted the aircraft carrier into the role of the premier naval asset. After the loss of Force Z, the Battle of the Coral Sea and Midway, the battleship was obsolete ... much to the annoyance of the old school battleship advocates.
An Aircraft Carrier is everything, You have access to the Land, Sea, and Air.
Future Carrier (if it gets built)
Historical Carrier (WW2)