The 15 Schengen countries are: Austria, Belgium, Denmark, Finland, France, Germany, Iceland, Italy, Greece, Luxembourg, Netherlands, Norway, Portugal, Spain and Sweden. All these countries except Norway and Iceland are European Union members.
There are a few more Schengen countries now, such as Poland and Czech Republic.
As a consequence, when I did Poland to France (and vice versa) recently, there was no border control and there was thus no passport stamping.