Percale refers to the type of cotton weave used to construct the sheet and consists of a traditional criss-cross pattern of one thread over and one thread under.
A fitted sheet is different in that the corners are elasticated so that it is not necessary to tuck the sheet under the mattress as the elastic will serve to keep it in place. It is not possible to use a fitted sheet as a top cover because of the elasticated corners.
Everything about the sort of cloth you pick for your bedding comes down to personal choice. Consider the fact that it is what your skin is resting on each night.