Pay per click search engines like Overture are designed around a very simple concept - the more you are willing to pay for a clickthrough, the higher up the search engine results you will appear.
Basically, it's a real-time auction between you and your competitors for the top few positions (which receive the vast majority of the click throughs).
For example; if your competitor is bidding 50p for the term "baked beans" and he is in the top spot, a bid of 51p will put your site into No. 1 and push him down to No. 2.
Of course, another competitor can come in with a 52p bid and push you both down - that's bidding!