Haram = any activity prohibited in Islam which is against the moral values and principles.
Bida'h = a new practice that people say is a part of Islam, but in fact is not.
I know there are many kinds of Haram activities. I have studied the major sins of Islam. But Bida'h is not listed. Yet Bida'h is a sin. Bida'h leads to shirk. And we know shirk is the biggest of all sins. So is Bida'h a major sin or minor sin?