This term is made up of two parts:

Enjo = support/help somebody
kosai = association/dating

Most of the schoolgirls who do this will vehemently protest that it is not prostitution, however I fail to see the difference. Older men "buy" sex from young girls. I actually think this is probably worse than most prostitution because it involves young school girls and much older men. Most of the girls themselves think of it as a "victimless crime", they are getting nice things in return for sex and they don't believe that they are hurting anyone. However the number of violent enjo kosai related incidents are increasing and with more and more young girls voluntarily going off alone with older men, it doesn't look like it will decrease any time soon.

This issue is examined in the live action movie Love & Pop, which was directed by Hideaki Anno of Evangelion fame.