How does advertising on the Web affects search results?

Question by CrazyNinja: How does advertising on the Web affects search results?
I want to know how advertising on the Web affects search results and the other ethical dilemmas associated with it.

Should an ad on a page be used to index that page in a search? Why or why not?

What about Web page authors who add invisible tags to increase the visibility of their page in search results? Can you find an example?

Thanks!

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