Wikipedia is a pretty spot-on source for math. In addition, you can check out this page:

Peter Suber, "Infinite Sets"

And you can find free online textbooks with chapters devoted to set theory. Here are two such examples:

http://ramanujan.math.trinity.edu/wt...L_ANALYSIS.PDF

http://www.cosc.brocku.ca/~duentsch/...ethprimer1.pdf