The * union* (written *&cup*) of two sets
is the set of elements that are members of either set.

(union '(a b c) '(a c e)) -> (b a c e)

The * set difference* (written *-*) of two sets
is the set of elements that are members of the first set but not the
second set.

(set-difference '(a b c) '(a c e)) -> (b)