What exactly this money making thing is all about?

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many of us here are asking about how to make money online..even i asked this 2 weeks ago from this community on mylot and many of people helped me with the good sites and i thank them all..but one thing i am totally unable to understand and which is still preventing me from believing in such sites is what is the benefit they get in return..like if we are clicking ads on some site,or clicking on links in our mailbox so called paid mails(i have never read what is in them),or sign up offers which i came across on a site,even blogging or posting can make you earn money..but how everything goes?..i mean what the site owners get in return..?why they want to pay us for clicking ads or doing anything like that?..if they want advertisement..i don't think anybody reads ads that seriously..i don't do that..i just click and watch some other site or do anything else..i am not criticizing any site and i just want to know..what all this actually is..hope somebody knows it..please do tell me if anybody knows that..thanks..