I'm not sure about your relationships, but my girlfriend is definitely a humper. Sometimes we joke about it (she insists she can't fall asleep unless she humps me.
Well, by definition, dry humping is nothing but dry sex. It's a kind of non penetrative sex or outercourse, where your partner and you graze each other's privates.
My girlfriend and I dry hump all the time and it's awesome, I know she's really not getting anything but I have an orgasm all the time when we do it. Some we do it.