Set Membership
  definition - symbol ∈ means “is an element of”...
                     - statement x∈A means that x is an element of the set A.
                     - symbol ∉ means " not an element of "......
                     - the statement  x∉A means that x is not element of set A.
   example:-    
                  A = {♢,♡,♣,♠}, then ♡∈A but △∉A (where the symbol ∉ means “not an element of”).

Empty set (null set)

   definition  - is set with no element...

                        - symbol {} or ø..

   example:-

            A = {2, 6, 8} and B = {3, 5, 7}

            Let C represent the intersection of the sets A and B.
            C = {}, a null set, because there is no element common between the two sets.