An About.com reader writes: "When does foreplay stop and intercourse begin?"
If you're looking for dictionary definitions, then this question is relatively simple to answer. Dictionaries, and most health texts that take a medical model approach to sexuality define intercourse as as a sexual activity involving penile-vaginal penetration. By this definition intercourse is something that only two people can do, one must have a penis and the other must have a vagina.
So the short, and least interesting, answer to your question is that foreplay stops when there is penile-vaginal penetration penetration, which is the point at which intercourse begins.
But what if penile-vaginal intercourse isn't the thing you do the most, or at all? What if there's no penis or vagina in the mix? Or what if there is, but it's not what you like to do more, or not what you think of as intercourse? The good news is that there is really no reason for you to subscribe to someone else's definitions of sexual activities. For example, as a sex educator I talk about intercourse as sexual activities that involve penetration of the vagina or anus. I don't specify what or who is doing the penetration. I'm not sure why that's necessary.
The words we use for sexual activities just need to be clear enough for us and our partners to understand what we're talking about, and so we can all make sure we understand what (or who) we're getting into, and what, if any, risks we are choosing to take. Scientists might prefer if we had sex in a way that makes it easier for them to study, but that seems a bit more responsibility than we need to take.
There's something that your question exposes which I think is worth considering. One of the pitfalls of thinking of sex as the same as intercourse is that it imposes a harsh linear structure to our sex play. You ask when one thing ends and the other begins. This is a sort of false understanding of how sex can be. It's the picture we get from pornography and mainstream media of course. And it's the story we're told in sex education class (if we're told anything at all). Sex begins with touching, progresses to foreplay, and ends in intercourse and orgasm. It's a nice story, I suppose. But it's a fairy tale.
Being sexual with a partner may start one way, veer off in a different direction, come back to the original plan, stop in the middle, start again ten minutes later. It might end in orgasm or not. Sex can take two minutes or three hours, depending on what you're doing (no one is going to engage in intercourse for three hours without a break, it wouldn't likely be pleasurable, and it wouldn't really be possible for most humans). For lots of people sex doesn't include intercourse. If you're having sex to feel pleasure and/or make your partner feel pleasure, then that's the only goal you need to consider. There aren't any check boxes, and sex isn't successful if you make sure you do everything "correctly", in the "right order", and for "long enough." In fact like many sex educators I would suggest that the more expectations and rules you put on your sex life the more difficult you'll find it to be creative about giving and receiving sexual pleasure.
I'm not sure why you were curious about this particular question, but my suggestion is that you work to forget any rules you think there are to how to have sex, and instead ask yourself what it is you want to do, what do you think will feel good for you. If you're doing it with a partner, you need to ask them too.