Click thru marketing - a form of marketing which thru the use of a listing provider like Overture, charges a per click fee for each time some reaches your site by clicking a link that they have provided to other search engines. The fee amount is fluid based on other companies or web sites that are competing for the very same term. Each competing entity bids and can be out bid for a spot.
Generally speaking only the top 3 positions in the bidding are considered for cross engine display. This means that if you are not paying or have not bid your link to your site up to at least the third highest bid, you will not be displayed in the search engines that have an agreement to do so with your listing provider (in this example Overture).
Because many businesses could be competing for the same prime phrase or keyword, the top three bid spots could cost several dollars per click thru. This being the case, it is absolutely imperative that your product or service be one that will generate enough revenue to sustain your listing.