What is the difference between clicks and impressions

You will see that some advertisers sell you 1000 impressions. The cost for 1000 impression is CPM (cost per 1000), or for the seller Price per M (one thousand in latin ).
  Buying impressions is not effective, and not recommanded, because your ad can be posted somware in the bottom of the web page, and when somebody open that what page, an impression is accounted and paud by you, but the visitor cannot view your ad because this ad is at the bottom.
  So, the right way to be sure that somebody saw your site is to buy clicks.
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The Art of SEO: Mastering Search Engine Optimization (Theory in Practice)  Stephan Spencer