Bids are placed on phrases that would be used to find products or services such as 'football strips'. Adverts are generally shown on the right hand side of search results as shown in this example:

http://www.google.co.uk/search?q=football+strips

When a search engine user clicks on an advert they are taken to the most appropriate page on the advertiser's website. Although adverts show every time a targeted phrase is used in a search, costs are only incurred when a visitor is delivered hence Pay-Per-Click.