what is the difference between functor and function? As it is, what is the difference between functor and function? As far as I know, they look really similar.
And is functor used in set theory? I know that function is used in set theory.
Thanks.

A simpler explanation:
Functions map arguments to values while functors map arguments and functions defined over the arguments to values and functions defined over the values, respectively.
Moreover, the functor mappings preserve function composition over the functions on arguments and values.
Briefly, functions map element while Functors map systems (=elements+functions over them).
