i think the distinction is between ads that google sells vs software infrastructure for 3rd part ads (ad tech). ie the revenue's from google's ads on search, maps, youtube, etc are still huge and important -- that's google's advertising business, the eyeballs they sell, if you will. ad tech is google's business brokering ads on other sites, helping others sell their eyeballs.

at least this is how i understand it currently, but i don't know very much. happy to be corrected by someone who does.