Difference between mild cam and big cam?

What is a mild cam, and what is the difference between it and a big cam? I've heard two things: either mild cams have a short duration or keep the valves open longer, and those two things seem to conflict each other. Can someone define it and tell me the difference between it and a big cam? (I wanted to know if I could get one on for a 1970 Plymouth Satellite with a 318.)