Currently, the Schengen area consists of 26 European countries Belgium, Czech Republic, Denmark, Germany, Estonia, Greece, Spain, France, Italy, Latvia, Lithuania, Luxembourg, Hungary, Malta, Netherlands, Austria, Poland, Portugal, Slovenia, Slovakia, Finland and Sweden, along with Iceland, Liechtenstein, Norway and Switzerland.
Being part of the Schengen area without internal border controls means that you can visit these countries on a single valid schengen visa.
These countries do not carry out border checks at their internal borders (i.e. borders between two Schengen states); carry out harmonized controls, based on clearly defined criteria, at their external borders (i.e. borders between a Schengen state and a non-Schengen state).
The EU states that are not, or not yet part of the Schengen area, are Bulgaria, Croatia, Cyprus, Ireland, Romania and the United Kingdom .This means that a flight from one of these states to a Schengen state is regarded as an external flight and is subject to border checks, and a separate visa of the respective country other than Schengen visa.