I've never really been into Hockey before...?

But, I've really gotten into it this year mainly due to our new team, the OKC Barons. But I'm a little confused as to what constitutes a "hitting from behind" call. I've seen some that were pretty clear and some that were questionable, but what does the league rule state? In that what makes it go from a minor to a double minor to a major, or is that just based on the severity of the hit and what the ref thinks?