This whole concept is relatively new, and nobody quite knows how it will end. It had its major beginning in 1999. In 2000 and 2001 there will almost certainly be a lot of new companies jumping into the competition, and most of them probably won't survive. Some of them are. Many are for real.
Advertisers are willing to pay big money to get people's attention.
They pay well for companies to put ads where you will see them. The companies are willing to share some of their ad revenues with you, if you will look at the ads. All you need to do is give them a little space on your computer screen. It is free to sign up for any of these, so there's no risk. If you spend several hours a day online, you ought to sign up for several of these.