If you have the tabbed inbox enabled (i.e. Social, Promotions, Updates, Forums), then Gmail will insert ads into the non-primary tab that look almost like regular emails. If you have the tabs disabled so that all your messages show up in the Primary inbox, you don't get ads.

To be fair, they are only showing it in the “promotions” tab, which is where all the other junk mail goes to.

There are ads on the internet?

I think I've even seen ads out of the internet!

They show up before you finish setting up the browser and installing the extensions required for daily usage.