"Spam" vs. "Junk"

I notice that Apple Mail has both a “Spam” and a “Junk” folder. What’s the difference? I thought the two terms were synonymous. In fact, when I look up “spam” in Mail Help, it lists a couple of entries referring to “junk” mail, but it doesn’t have a separate category for “spam.”

What’s more, when I’m going thru my Spam folder, unlike in my Junk folder, I don’t see any way to mark mail as NOT being “spam.” Unlike in the Junk folder, it doesn’t have any button you can click to mark it as being okay. Advice appreciated.