Heresy is the expression or following of a dogma, doctrine or religious belief that is different to the one mandated by the powers that be -- often the Catholic Church although the various Protestant powers of the reformation in turned branded many Catholic doctrines as heresy.
So, being a heretic really depended on what side of the argument that you are on.
- Lollardy, or the following of the teachings of John Wyclif. This was closely related to the teachings of the Hussites after Jan Hus.
- According to Mary I of England, Elizabeth I was a Heretic. According to Elizabeth I, Mary I was a Heretic