What is the difference between a BOT and A TRAFFIC EXCHANGE?

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I stumbled in a forum lately and the members were talking about a certain site which gives you an extra income if you bring unique impressions on your page. Now I know that there are other sites that gives traffic to certain websites but what I did not get is the prohibition of the "BOT" thing mentioned and not the "TRAFFIC EXCHANGE". Can anyone please enlighten me what's the difference of these two stuff?