There are different types of ads. Some do generate revenue just for impressions, or anytime someone sees it. Others generate revenue when they are clicked on. Still others only generate revenue when they result in a conversion. There are also some hybrids. I'm sure many of the ones we are seeing are based on impressions, so the advertiser would pay X amount and receive Y number of impressions. They will get reports on clicks, but if they aren't under a pay per click model, they won't pay for each click.
What's bothered me about the ads I've been seeing is that I'm seeing a lot in an Asian language. I seem to recall opening some message about something foreign like that, though I can't recall anything more specific than that, and all of a sudden I'm seeing a bunch of ads in that language. It's just a function of the algorithm that tries to present you with relevant ads based on your history, but it's just irritating when it gets it so very wrong.