In 1995, an initial agreement was signed in the town of Schengen (Luxembourg) between 5 countries to eliminate common borders and allow all kinds of free movements between their territories.
Until today, there are already 26 countries within this treaty, giving name to what is known as the Schengen Zone or Schengen Area.
What does this territorial organization consist of?
For practical purposes, it is an agreement that allows the free mobility of people, goods and services between all the member countries who are inside.
But it is very important not to confuse the Schengen Area with the European Union, because although many countries are part of both groupings at the same time, this is not the case with all of them (besides implying great legal differences).
So why is the Schengen area so important?
Among other things, for tourism and travel: the requirements and criteria for entry and the granting of short-stay visas are common throughout this region.
So, if you come from a non-European country (e.g. Latin American), you could visit any country within the Schengen Zone with complete freedom.
For example, you could enter through Spain, and visit Greece, Italy and France freely.
Generally, this would happen without any border control when traveling through each country, although it is true that recently some countries have started to impose extra controls after the terrorist attacks that took place during the last years, and the massive movements of refugees moving to these countries.
To summarize, any movement between countries that are inside the Schengen Area is equivalent to moving within the same state.
