Clean vs. Dirty Hits in The NFL

Football is a very competitive sport with a lot of physical contact, but some of the contact, are judged by if the hit is a clean or dirty. Most of the time the hits are often considered clean but what is defined as a dirty hit. In football i don't think there's no such thing as a dirty hit, because football can be violent sport and you're suppose to hit the other opponent as hard as you can. I understand that getting a cheap shot on someone away from the play or when someone gets called for a helmet to helmet hit. So what's really a dirty hit in football?