Before delving into it lets look at the very basic philosophy, for which the companies/
agents/ Market Research Companies/ Individuals pay you. The simplest logic is through referral programs. The people, who browse your site glance the other sites too, whose links/ banners are located at your site. When they visit the sponsor's site from your site, then you are entitled to a payment. Google Adsense is one such typical referral program, but then it is just one basic step.
Now in some cases, they make it a point that the money will be paid to you, if and only if the visitor purchases goods or services from their sites. Then in such a situation the amount paid is also higher.
The world of chain marketing has increased the true value of referrals, where you can get even life time benefits, once you refer the sponsors to some buyer and you keep on getting paid for each occurrence of purchase either by your referred customers or anybody under that hierarchy.
There are various ways of referrals. Some are like Survey and Earn, where companies pay to take the survey; Read email and Earn, where companies pay to read e-mails; Surf and Earn, where companies pay to surf the Internet