Heresy: Opinion at variance with the authorized teachings of the Church. Four elements constitute formal heresy:
1. You have had a previous, valid, baptism
2. You publicly profess that you are still a Christian (otherwise you are an apostate, not a heretic)
3. Outright denial or doubt regarding a truth taught by the Church
4. This must be a choice for which you are morally culpable (responsible).
The fourth point is key. People make mistakes all the time and theology is a difficult subject. You might very well think the Church teaches one thing when, in fact, it does not. This means you are in error, but it does not mean you are a heretic. A heretic persists in error even after being corrected by someone with authority (ie: a bishop).
Special Note: Just because someone is a heretic doesn't mean it is always prudent to call them one. For some reason I feel compelled to write that.