Greg discusses the distinction between heresy and bad theology.
It is not unusual for a Christian, when he or she hears some doctrine that they disagree with, to label it “heresy.” Heresy is a very particular word meaning that whatever is in question is so far off of accurate theological teaching that the person who believes it is no longer a Christian. Heresy disqualifies you from salvation, generally speaking. That's the way I think the word ought to be used. It is not simply bad theology. There's lots of bad theology that doesn't qualify as heresy that might instead be considered a heterodox. Heterodox means not sound but not off the reservation.
You have things that are bad theology, meaning that it is not accurate but it really is a legitimate difference between people. People believe that you ought to baptize through immersion and that's the only thing that is appropriate for baptism. Others say sprinkling is legitimate. They can't both be right as stated. One could be called bad theology, but I hope you see that probably this is not really high on the theological pecking order. It might be wrong or heterodox, but not off the reservation.
Then you have heresy - teaching that is inconsistent with the creeds and classical teaching of the church. These things would relate to the person and the work of Christ, the work of the cross, the resurrection, the deity of Christ, the personality of the Holy Spirit, the Trinity, and things of that that order. You start deviating from those - that's heresy.
If we use the word “heresy” to describe merely bad theology that might be heterodox or just different from what other people think is right, then we water down the Word, and it can't do the work it needs to do when we really need the Word. So let's leave the word “heresy” for the kind of bad thinking that that takes somebody completely out of classical Christianity. Let's use “heterodoxy” to describe errant teachings within the pale, and let's just talk about the other things as mere differences of opinion that we can't both be right about, but are not that substantial in the big picture.