National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Numbers
Math Forum response to request for information on various number sets. Also defined and discussed are integers, natural numbers, whole numbers, irrational numbers, transcendental numbers, and algebraic numbers.
University of Utah
University of Utah: The Number System
This site from The University of Utah provides a definition of commonly used numbers including rational numbers, natural numbers, integers, real and complex numbers and more.