A true dry hump involves taking off all clothes except for one partner's pair of underwear, penetrating as far as the fabric barrier will allow, and doing a sort of hybrid grinding/thrusting motion until the man cums.
Dry humping is mostly commonly associated with teenagers, who either do it because they're scared of the potential consequences of sex or claim to be committed to keeping virginity. Many teenagers perform similar actions with more layers of clothing on, but this is actually considered grinding rather than true
dry humping.
Dry humping also takes place among couples, usually when either 1) the mood strikes and there's
no condom on hand or 2) the girl is on her period and doesn't want to
make a mess. In the former case, the man keeps his underwear on; in the later, the woman does. It can be a pleasurable
changeup.
1) "I'm so horny,
Kristin." "I know, Michael, but remember our
promise rings. We can dry hump but we can't have sex."
2) "I'm so horny, Kristin." "I know, Michael, but now that we're adults who no longer give a shit about our promise rings, you need to remember to bring a condom. We can always dry hump."
3) "I'm so horny, Kristin." "I know, Michael, but
I'm on my period. It won't be as messy if we dry hump."