The Schengen Area is a group of 26 European countries that have agreed to allow passport-holders of any member country to move freely within the area without border controls. The Schengen Area includes most of Western Europe, as well as some countries in Central and Eastern Europe.
A Schengen visa is a visa that allows you to travel to any of the Schengen countries for a short stay, usually up to 90 days. If you are a green card holder, you may need to apply for a Schengen visa even if you are a citizen of a country that has a visa-free regime with the EU. This is because the Schengen visa requirements are based on your nationality, not on your immigration status.