Add-on Interest
This site provides some explanation of add-on interest, provides some additional problems and some sample problems.
http://www.math.hawaii.edu/~hile/math100/consb.htm

Add-on vs. APR
Reading this site is almost like reading the fine print on a contract, but it does explain things if you have the energy to read what is there.
http://www.dsiagency.com/edu210.html

APR Reading
this site is about as much fun as reading the previously listed site, but if you take the time to read this you will be better informed about the APR.
http://www.amo-mortgage.com/library/interest/apr.html

Worse than Loan-Sharking
This is a useful site giving you some advice on proper use of credit.
http://credit.about.com/ library/weekly/weekly/aa042100a.html?once=true&