Promise vs. Engagement Rings

I am not sure that I understand this whole thing. You have promise rings, engagement ring and wedding rings. I get the impression that they are all different and I understand the engagement ring, it's to symbolize that you are engaged to be married. The wedding ring is to symbolize that you are married. But what is the promise ring? What are you promising? I just don't get it... And another thing, what is the promise ring etiquette? At what age is it appropriate to give a promise ring? How much should you spend? I just don't quite know what to think about this...