When you created your ads in the AdSense setup procedure, there was a section that said “More options” on the 2nd step of the process where you can choose your channel. Below is a screenshot of this section:
At the bottom, you can see a choice for “Alternate ads or colors”. This is in case there are no relevant ads for your site. I normally leave the setting on “Show public service ads”. This means that Google will show ads that are for charities, for example, and you will not receive anything if any users click these ads. (I tended to see a lof of Hurricane Katrina relief request.
You will probably find your site generating these sorts of ads a lot at the beginning. This is attributed to the fact that Google doesn’t quite know what ads to serve just yet. Don’t worry, they will disappear, and relevant ads will start to show. If you see these ads for a long time after you start using AdSense, it would be advisable to send off a “request for help” to the Google AdSense team. They might be able to respond and tell you if there’s a specific reason you are seeing these sorts of ads.