Search Engine Watch has an interesting article on PPC "drafting" and "hijacking" that is worth checking out. See the article here.

For those who want the Reader's Digest version - here are the definitions:

Drafting - where an advertiser conducts a search marketing campaign and a competitor takes advantage of the traffic by bidding on related terms

Hijacking - when a competitor outranks an advertiser's campaign by outbidding them

But seriously, the full article is like 13 short paragraphs long, so read it already!