What is the unforgivable sin?

I was reading another question, and there are a few different answers. How am I supposed to know whether I've done it or not? I seriously don't mean to mock, I just want to know if my chances of going to heaven are over and I should just enjoy earthly life because that's all I have left to look forward to. Is it blaspheming the Holy Ghost (and what if an inadvertent thought crossed my mind after hearing that?) or is equating God with the devil, or what?????