Pay Per Click
Bottom image to the right shows a web search with paid advertising in the off color section on the top and down the right coloumn. The Search Engines track visitor behavior and charge the advertisers a fee for each visitor that clicks on those ads. These tend to operate as an auction and fee can range from trivial to extreme. There is now question that this approach will drive traffic to your site.
These ads are also placed on sites all over the web. Your favorite blog, popular ecommerce sites, all the major search engines, news sites mobile app, etc.
Google is, of course the number one in the business and you can get started here. Another provider is XFinity ( Comcast ). AOL does a huge trade in on line advertizing. Many individual web sites offer advertising opportunities, Facebook among them.