Can anyone tell what is the public service ads of google adsense?

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I am a new adsense publisher and run adsense ads in my blog. I have noticed that many times when I visit my blog I have shown public service ads by google. What it means? Is public service ads pay more/less from the other ads? I have searched about it in adsense terms but not found. Plz help me if you know what is it. Also please visit my blog & comment about it. Might be you will find something interesting which you never experienced before. Its url is in my profile.