Rational and Irrational Numbers
Even though this site was written for an elementary age group, it does contain some useful information.
http://www.themathpage.com/aPreCalc/rational-irrational-numbers.htm

Ask Dr. Math
Dr. Math answers the question in this section, "What is the difference between rational and irrational numbers?" This can introduce you to a very useful site... one in which you can ask your math questions and have a real, live person respond.
http://mathforum.org/library/drmath/view/58727.html