What is the diffrence between 'Per Impressions and Clicks"

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Hi All, I was checking the google adsense account and found that the impressions and clicks are diffrent. My concern is that I forwarded my blog url to my friends to check or any suggestions. They must have clicked to open, but the account shows only impression. please help me to understand. my blog url- need suggestion from you all. http://stayfit-dhusa.blogspot.com/ Please give your blog url so that i can add it to my list. thanks you :)