Recall that a set is a collection of elements.
Given sets A and B, we can define the following operations:
Operation 
Notation 
Meaning 
Intersection 
all elements which are in both A and B 

Union 
all elements which are in either A or B (or both) 

Difference 
all elements which are in A but not in B 

Complement 
(or ) 
all elements which are not in A 
Example 1:
Let A = {1, 2, 3, 4} and let B = {3, 4, 5, 6}.
Then:
= {3, 4}
= {1, 2, 3, 4, 5, 6}
= {1, 2}
= {all real numbers except 1, 2, 3, and 4}
Example 2:
Let A = {y, z} and let B = {x, y, z}.
Then:
= {y, z}
= {x, y, z}
=
= {everything except y and z}