Basics about Sets
This site (with Links ) discusses the notion of sets.
http://www.cenius.net/refer/display.php?ArticleID=setnotation_ref

Equivalent Sets
Some definitions and examples are given.
http://www.math.uic.edu/~lewis/las100/equiv.html

Arithmetic
What is arithmetic? What are the basic foundations of arithmetic and what does it have to do with sets. This is interesting reading in light of the various sets of numbers which are reviewed in this section, and ties together many of the chapters in this text.
http://www.encyclopedia.com/html/a1/arith.asp

A History of Set Theory
This site (with many Links) give some of the background and history of this topic.
http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Beginnings_of_set_theory.html