https://en.wikipedia.org/wiki/Visa_policy_of_the_Schengen_Area

The visa policy of the Schengen Area is set by the European Union and applies to the Schengen Area and to other EU member states without the opt-outs enjoyed by Ireland and the UK.^[1] If someone other than a European Union, European Economic Area (EEA) or Swiss citizen wishes to enter the Schengen Area, Bulgaria, Croatia,Cyprus or Romania they must have a visa or be a national of a visa-exempt country.

The Schengen Area consists of 22 European Union member states and four non-members who are members ofEFTA: Iceland, Liechtenstein, Norway and Switzerland. Bulgaria, Croatia, Cyprus and Romania are not yet part of the Schengen Area but, nonetheless, have a visa policy that is based on the Schengen acquis.^[2]

Ireland and the United Kingdom opt out of the EU’s visa policies and instead operate their own separate visa policies, as do certain overseas territories of EEA member states.

https://www.axa-schengen.com/en/visa-info

The Schengen Treaty provides for the removal of the internal border controls between the Schengen member states. The Treaty provides for joint rules for controlling the external borders of the Schengen Area. The Schengen Treaty also provides for the same visa obligations and rules for obtaining a Schengen visa. The aim is to enable the free movement of persons within the European Schengen Area. The Schengen countries have established a set of joint rules that have resulted in:

• The harmonisation of the visa obligation and rules for visits of less than three months
• The disappearance of internal border controls within the Schengen Area, which means there are no longer any passport checks in place at the internal borders. Beware as each member state of the Schengen Area has its own rules as to whether or not travellers are to be in possession of travel documents and/or a passport
• The segregation wherever possible at airports of persons travelling within the Schengen Area and persons coming from outside of the Schengen Area;

Please note: the Schengen Area is not identical to the European Community (EU). Even though the United Kingdom and Ireland are both EU member states, they have decided to opt out of the Schengen Area, preferring to maintain their own border controls. Which means that if you are intending to travel to either of these two countries, combined with a visit to any of the European Schengen countries, you will need to observe a separate procedure.

If you are intending to stay in any of the Schengen countries for a period longer than three months, the Schengen visa no longer applies. Instead, you are required to observe the specific rules of each separate country.

In 2001, the “Council of the European Union” adopted a list of residents of third countries that are subject to the Schengen visa obligation for short stays of less than three months.