Whats the difference between a

In the days of sailing ships the formal requirement for a vessel to be a ship was that it had to have three masts (each made up of more than one pole) and that each mast had to have at least one square sail on it. Any other rig and it was not a ship. It could be a snow, lugger, cutter, schooner, ketch, brig, brigantine, polacre, barque, barquentine etc. etc., but not a ship.

Wow, so many different answers. If it's a ship because it can carry a boat, then is something like an aircraft carrier the only real ship type as its capable of carrying most other ships? Do those ships then become boats?

The largest vessel normally carried on another vessel would be a landing craft, for landing troops, tanks, supplies and other equipment on a hostile shore. They are carried on a large vessel, usually called an assault ship. The floating dry dock can be very large, large enough to carry a very large ship for repair. Although it floats, it is not a vessel, as it is not self-propelled, being reliant on tugs to move, so would not be classed as a ship itself.

The largest vessel normally carried on another vessel would be a landing craft, for landing troops, tanks, supplies and other equipment on a hostile shore. They are carried on a large vessel, usually called an assault ship. The floating dry dock can be very large, large enough to carry a very large ship for repair. Although it floats, it is not a vessel, as it is not self-propelled, being reliant on tugs to move, so would not be classed as a ship itself.