The Schengen Area is comprised of 26 countries out of which 22 are part of the EU and the other 4 are part of the European Free Trade Association (EFTA). These countries have agreed to allow free movement of their citizens within this area as a single country.
The Schengen countries are Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland, and Liechtenstein.
However, regarding the citizens of non-members of the Schengen Area, there is a mandatory visa requirement to enter the Schengen Zone for some specified countries. India is one such country.
In case one intends to visit only one Schengen country, then an application for the visa has to be submitted to the Embassy or Consulate representing the particular country of your destination.
However in case the individual intends to visit more than one Schengen country, then the visa application should be submitted to the Embassy or Consulate representing the country of the main destination (the country of longest stay).
Also, in case the individual intends to visit visiting more than one Schengen country with equal stay duration, then the visa application should be submitted to the Embassy or Consulate representing the Schengen country, in which one will enter first.
It may be noted that Unite kingdom is not a part of the Schengen Area. Therefore a separate visa is required for entering UK. However, in case the individual wants to visit France also together with UK, then UK visa as well as, Schengen visa for entering France will have to be obtained.