Schengen Countries are countries that have signed an agreement, currently between 26 European States, to remove immigration controls to travel within their collective territories. This means entry into one Schengen Member State gives free access to all the other Schengen states.
If the visit includes more than one destination, the visa has to be applied to the Embassy/Consulate of the country that is the main destination of the journey (length of stay or the purpose). If no main destination is determined, the visa application should be submitted to the Schengen country of first entry.