Heresy
The Arian Controversy
Helpful introduction to the early heresy and a collection of useful links, by your About.com Guide to Ancient/Classical history, N.S. Gill.
Catholic Encyclopedia: Heresy
Substantial explanation of the origins and degrees of heresy as defined by the Catholic Church as well as its role in history, by J. Wilhelm.
Heretics, Heresies and the Church
Helpful overview by Steven Kreis at the History Guide covers the established Christian view and some of the belief systems that deviated from the norm.