Have you heard about Google AdWords?
OK, this time I'm going to explain a bit about it.
First of all, you know as well that Google is still #1 Search Engine in net, leaving Overture at second place. If you type a word or a phrase in Google's search box and press "enter", a large number of links will appear in front of you, related to subject you've typed earlier.
Take a look at right hand side of main page. There should be another group of links complete with their descriptions, right? Only, these ones are much smaller than those main link results. What is difference?
Do you see subtitle.. Sponsored Links..? Those link results on right hand side is called Google AdWords. People bid a certain price to Google to put their links on there everytime visitors search for related keywords. Those advertisers are only paying for numbers of click-throughs to their links. This kind of advertising is called Pay Per Click (PPC) advertising.
In order to reach a higher position in link results, Google has set up a unique formula. Let's say you want to put your link in Google AdWords and you bid for "flowers" as your main keyword. You are not only one who bid for that keyword. There are lots of people who also bid for it, since they also want to promote their "flowers" sites!