Exactly what it says on the tin - you the client - pay each time someone clicks on your link.
Simply put you can divide the search engine results pages presented by the major search engines into organic or natural results and then pay per click.
Organic or natural results are produced by the search engine crawling the web and finding sites to present. These can change at any time and the web site owner has no direct input into how high - or low - these web sites are presented.
The opposite in almost every sense is true of pay per click. They appear in a different place - mostly at the top of results or on the right hand side of the screen. Usually, they are identified with the words 'sponsored link'.
They change when your budget runs out - or when someone else competes with you by offering to pay more money for each click on the link. So the client does have some control over their positioning - provided that the money is available to do this.
Webcertain runs optimisation campaigns alongside pay per click campaigns. We believe that wise clients use these too different tools strategically. In other words, in a planned way and for different specific purposes.