Schengen Agreement is the name of an agreement between European countries that was started in the late 80's. The plain objective of this agreement is to remove borders between countries and to ease migration among them. Although there were many hurdles in implementing this agreement at the beginning, in 1985 Belgium, France, Luxemburg, Germany signed Schengen Agreement whereby they agreed to gradually remove the control on the borders line that separates those countries. Between 1990 and 1995, more and more members of the European Union started to join the agreement. With this agreement, EU citizens do not need to show their passports when they cross the borders. Also, if someone with visa from a Schengen country wants to enter other Schengen countries, they are free to do so without applying for another visa.
Now, 22 countries have signed this agreement. Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Poland, Portugal, Slovakia, Slovenia, Spain and Sweden. Iceland and Norway that are not part of the Union also signed this agreement. In contrary, there are also some EU members that have not signed this agreement, such as Bulgaria, Cyprus, Ireland, Romania and UK.
In the borderline area between countries under Schengen agreement there is no guard post to check passports anymore. What we usually find are only welcome signs, marking that we have passed the borderline between two countries.
With this agreement, European citizens under Schengen don't have to spend their time providing or showing passport when they visit other EU countries. Everything looks easier under this agreement. This helps job-seekers find jobs in other countries and definitely ease tourists during their travel. It has been shown that this agreement increases migration rate in Europe. There is nothing surprising with this argument because we know there are countries that have better living standard than others (e.g. higher minimum wage).
However, this agreement also has some downsides. For criminals, terrorists, illegal drugs distributor, this agreement helps them to hide themselves in another country and allows them to have a regional, cross-border operation. As a consequence, a more active cooperation and coordination between countries is needed. Another effect of this agreement is that EU countries tend to put more control on foreign nationals.
From a statistic standpoint, this agreement has contributed in approximately 8 millions illegal immigrants scattered in the continent. Logically, it is easier for illegal immigrants to move around in the continent due to the loose control of the borderlines.