The Schengen area is constituted by 26 European nations with similar interest and have all come to terms as well as agreed to abolish passport or visa requirements to enter into the common area or any other form of border control within the area.
Basically, Schengen is a concept of free movements which entails free entry and exit of any country within the member states.
Thus, if you are travelling from one Schengen country to another, you do not require any visa, immigration controls or any other requirements that was previously mandatory to enter into these countries.
The Schengen area member countries include:
Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Spain, Sweden, Switzerland, Liechtenstein, Poland, Hungary, Iceland, Latvia, Luxembourg, Portugal, Slovakia, Slovenia, Iceland, Italy, Lithuania, Malta, Netherlands and Norway.
However, if you are not a passport holder of any of the countries in the Schengen area, (Non-Member), you will require a visa to enter into any of the countries but of course there are some exception for some countries who will not require any form of visa to enter into any of the Schengen countries.
For Nigerian passport holders, you will require a visa to enter into any of the Schengen countries as well as also a
transit visa if you are transiting through any of the Schengen countries.