We briefly talked about it in my world religions class, but we didn't get a lot of context. I know he was very upset about indulgences (people paying a certain amount of money to be forgiven for a given sin), but I don't understand why. And were indulgences the main reason he left? Did he know he would be excommunicated when he wrote his 95 Theses?
You may be interested in an answer I previously gave to a similar question.