they built during the war by converting existing merchant ships into small "mini-carriers". They carried a very modest number of planes and were not very fast. So, their main duty was to serve as protection for convoys or groups of merchant/support ships. They served a very important function during WWII in the Atlantic and even more in the Pacific. One of them, the U.S.S. Guadalcanal, helped in the capture of the U-505 which you can tour today at the Museium of Science and Industry in Chicago (highly recommended).
this may be of some local interest