Main differences between a Set and a Map in Java are...

Duplicate Elements : A Set does not allow inserting duplicate elements. A Map does not allow using duplicate keys, but it allows inserting duplicate values for unique keys.

Null values : A Set allows inserting maximum one null value. In a Map we can have single null key at most and any number of null values.

Ordering : A Set does not maintain any order of elements. Some of sub-classes of a Set can sort the elements in an order like LinkedHashSet. A Map does not maintain any order of its elements. Some of its sub-classes like TreeMap store elements of the map in ascending order of keys.