The first US carrier was the Langley (CV-1) was a converted collier (coal refueling ship). The Lexington (CV-2) and Saratoga (CV-3) were converted from battlecruiser hulls after the naval conferences in the 1920s. Battlecruisers had the same large guns as battleships, but they sacrificed armor protection to gain speed. They couldn't take hits from comparably armed enemy ships.
I found this informative article contrasting the classes and explianing the development. I hadnt realized that the Hood was a refitted battlecruiser, her explosion illustrating her origin.