Google says that they pick up on keywords... and I've noticed it pretty much seems to be the case. I haven't seen any car repair ads or anything LOL.
It hasn't worked out so well for me. I have written too many articles at my site that have the word "squirrel" in them. In an article about backyard birding, I talk quite a bit about how to live peaceably with your squirrels and how I don't understand why everyone goes to war with them. I love the little beasties.
So guess what most of my Google ads are about? How to electrify your birdfeeder, buy a "twirl-a-squirrel" feeder. Animal abuse at its finest.
Now I need to edit my adsense preferences to exclude all the squirrel abuse products !
So it does work, tho not always exactly as intended