Polynomials
This site distinguishes numbers (constants), variables, and algebraic expressions. We will consider the topic of polynomials in the following chapter.
http://faculty.stcc.mass.edu/zee/newpage22.htm

Variables, Expressions, and Equations
This site introduces some vocabulary and notation in using variables. Some of the topics of this site will be discussed in the next chapter.
http://www.riverdeep.net/math/destination_math/dm_tools/algebra1.1/algebra_I.1.13.jhtml

Leonhard Euler
Did you notice the Historical Perspective at the beginning of this section? Leonhard Euler (pronounced "Oiler") is one of the greatest mathematicians in history. If you want some additional information, you can check out this site.
http://www-history.mcs.st-and.ac.uk/~history/Mathematicians/Euler.html