AdWords ads are seen whenever your keywords (and those of your competitors, both directly and indirectly) cause your ads to "enter the ad auction".
This auction is triggered whenever a keyword search is carried out, or a browser refresh.
The resulting ads delivering impressions (being shown on your device screen) can vary widely depending on a number [...]

Introduction to the Google Ad Auction

Our Chief Economist, Hal Varian, explains the AdWords Ad Auction and how your max CPC bid and quality score determine how much you pay for a click on Google.com.
So you put your AdWords bids up to raise your Ad position, and your AdRank (position) actually falls – what’s going on?
Google’s [...]