union

(clojure.set/union #{1 2 3} #{4 5 6})
=> #{1 4 6 3 2 5}

(clojure.set/union #{1 2 3} #{4 5 6} #{3 4 7 8})
=> #{7 1 4 6 3 2 5 8}

difference

(clojure.set/difference #{1 2 3} #{4 5 6})
=> #{1 2 3}

(clojure.set/difference #{1 2 3 4} #{3 4 5 6})
=> #{1 2}