The advertiser is charged a fee each time a person clicks on their advert so transferring them to the advertiser's web site. If the advert is displayed but is not clicked on, there is no charge on those occasions.
Google Adwords (Google) and Yahoo! Search Marketing (Yahoo, Alta Vista, Lycos, Wanadoo and MSN) allow advertisers to place adverts above and next to the free ('organic') results for a search. The adverts feature a link to the advertisers web site. Advertisers bid for the top spot on individual search terms and these can range from just a few pence to several pounds.
Use the Bids Tool available from Yahoo to research how much competitors are paying to advertise on relevant search terms.
Social Network sites such as Facebook offer the opportunity to display Pay Per Click adverts. Whilst these don't offer the attraction placing adverts in front of individuals actively searching, they do allow adverts to be highly targeted by social profile e.g. to females who are currently engaged in specific geographical areas etc.