Venn diagrams and Algebra of Sets
The operations of union and intersection, along with a Venn diagram is found at this site.
http://www.cs.uni.edu/~campbell/stat/prob2.html

Union and Intersection
This site compares and contrasts the operations of union and intersection. Some examples are provided.
http://www.themathlab.com/geometry/section2/union%20and%20intersection.htm