I know everyone thinks they know about this one. But let's make it clear.
Why this is a mystery to some people is beyond me. I'll grant you that very few boys are unaware of the idea that you can have sex without penetration - it's prevalent in modern popular culture, and most sexual "experimentation" probably involves non-penetrative sex, of a sort - but (as with most things) there is a line one can cross between sex sans infiltration and a mere erotic touch. As I don't think it's been stipulated (Wikipedia is typically vague), here's how I see it:
Penetrative sex, in my mind (and this actually aligns with the Wikipedia entry, damn!), involves the penis going into the vagina or anus, although I may also consider fellatio as a kind of penetration - and the same with cunnilingus, actually, especially considering how far one would care to insert one's tongue. Yet I wouldn't think of those as actual penetration - if we're talking in a sexual way... which, of course, I am.
But I digress.
Refer to the above diagram. Every one of those labelled is an erogenous zone. But that's not all - why not try this?
I've said it before and I'll say it again (if that doesn't make me sound too much like a grown-up). Everyone has their own sweet spot. Some are unique, some are not so much. Ears, wrists, thighs, feet, neck, shoulders, back... they can all be stimulated. The body is awesome sometimes.
So where do I draw the line; what do I consider non-penetrative sex?
This is where I have to think for a while. That time I incited an orgasm by licking a shoulder - was that non-penetrative sex? What about the circular stroking of a knee under a table that had the same effect? Well, no, I'm not sure if that counts. It's sexual excitement without actual cock-based penetration, but it's not the act I have in mind. The aforementioned Wikipedia article also mentions dry sex, which I've always thought of as sex with clothes on - I wouldn't consider that non-penetrative sex either. Although they are, like non-penetrative sex, not specifically foreplay.
And I want to make that clear too. Non-penetrative sex is not equal to foreplay. A young lady I used to attend my school's Christian union with once said, "there's foreplay and there's sex"... and she called me narrow-minded! Anyone with half a brain can work out that versions of stimulation which don't involve penis/vagina don't need to be a precursor to full sex. That's not how it works, soldier. If you're aiming to achieve orgasm (and you shouldn't; orgasms are fantastic, but you shouldn't start off with orgasm in mind; where's the enjoyment in the pleasurable bits in between?), and you both enjoy whatever you're doing, why should it be considered foreplay? If you come, surely it isn't! Jeeze!
Anyway, where was I?
I've always considered non-penetrative sex to be a premeditated act. You have sex - it's something you do. Well, I like to think of it like that. You have non-penetrative sex. Whether it's through use of fingers, tongues, and/or focused on any, all or fewer areas of the body, I think it should be an act you take some time over, like having sex. Yes, the definition will always be a little fuzzy. But there's a scene in David Lodge's novel Nice Work in which the main character and her S.O. are, as he puts it, "having non-penetrative sex," and he appears to be in no doubt how to portray that. It's frisky, exciting, stimulating, and that's what sex is all about.
So let go of the need to conform in every instance. Nothing needs to go anywhere 100% of the time; where's the variety if that is the case?