I'm not sure that there is complete agreement on the definition, but here's one: massage is the rubbing and kneading of body parts.  If she gets naked but only massages you with her hands and fingers, that's nude massage.  If she's massaging me, I want her to massage my private parts (privates), too. That's still just nude massage.  

If she rubs her private parts on my private parts, I think that is something also called a "stripper slide" and that is what I think of as "privates to privates".  

Body to body might when she (naked) rubs her body over yours, without necessarily getting her privates too close to your privates.  That often means she massaging you with her boobs: you can be face down being "tickled" up and down your back or ass; you can be face up and she can be on top of you face to face or her back to your face and be massaging you.  

Others, I am sure, will chime in with corrections or comments.  E.g., some people consider stripper slide a little risky as her fluids may be coming way too close to your urethral opening

ba-doom

But seriously chokes:

There is an obvious health problem with sliding an uncovered penis against a woman's labia and clitoris in terms of precum, etc.

This can be rectified by the use of a condom, or the rubbing can be elsewhere.

For example, I've had gals who enjoy grinding their genitals against my hip bone while my penis slides on their well oiled abdomen/thighs.  This keeps things fairly safe and interesting.