When you watch programming, you also see commercials. Those advertisers are paying that television station to air their commercials. It works the same with cable companies. The money you pay to your cable vision company pays to connect you to more channels but you still see advertising. HBO and Showtime doesn't show advertising, but you pay extra money to see it.
Public television works differently, but there are still sponsors.
Advertising is big business. It supports print magazines and newspapers, television, and now the Internet. So next time you see an advertisement realize that it is paying for the content on the page. The company providing the information wouldn't pay to produce the content and have a reliable way to get it to you, if they weren't going to get paid for it. Some companies have a Web site so that you can find out information about them online.