Once PR #3151 is merged, Google Ads (formerly Google AdWords) will be displayed on Steemit post lists, on the right side bar (below the "Updates Log"). These "post lists" are the Trending, New, Hot, Promoted tabs at the top, and your feed. Presumably the ad revenue will go to Steemit Inc. Currently, no ads will be displayed on the contents of posts.
Before there was a framework in place, and Google tracking activated, but no ads were displayed. This PR (authored by @rjungemann, who added the code that laid the framework for this previously) just places the ads in certain locations, and didn't actually have any significant code changes or development.
Here is a never before seen screenshot of the updated Condenser, with the Google ads. I took the screenshot on my own Steemit (actually Condenser, but that doesn't matter) instance, with the Google Ads changes. Unfortunately since my test instance hasn't been approved to show ads, I'll have to represent where ads will be with a blue rectangle. I also have to zoom out so you can see the entire ad:
Note that currently ads will not be shown for mobile users, as there is no space for an ad that tall on small screens. I suspect this will change in the future though.