To answer this question, one must look at the higher education institution (HEI) level. In practically every European country (and most other countries as well) students are collectivly represented in one way or another in the decision making process at their HEI. In most cases this means that students have the right to elect representatives to a student assembly; student body; student council; student (self)government, student parliament or student representation at the HEI. Regardless of the real name of this representative body of students, it is usually called a Student Union in the circles of ESIB.

In most countries these student unions have gathered to form a National Union of Students, to represent the local student unions both nationally and internationally.

This is not the case everywhere; in Austria for example all students are organized in the NUS, which then forms local bodies at the HEI level. In some countries, local student representative bodies don't have the formal power to organize themselves nationally and in some cases the bodies form external committees that can be organized nationally and in other cases students have to organize themselves individually in order to create an NUS.

Where student representative bodies are allowed to form NUSes (which is the case in an overwhelming majority of the European countries), they sometimes organize themselves mirroring the higher educational legal situation in that country. As higher education isn't organized on a federal level in Belgium or Serbia-Montenegro for example, there are NUSes representing the entities of those countries (ie Flanders and Wallony or Serbia and Montenegro respectively). In other countries higher education is formally divided in universities and polytechnics or public and private, thus giving rise to two or more NUSes in each country.

Regardless of the number of NUSes in each country, most NUSes represents the local student representative bodies and this is also most often recognized by the various national governments, either formally through legislation or informally through giving the NUS representative powers on a national level.

To summarize it all; the main difference between an NUS and any other national student organization is the fact that an NUS represents elected student representative bodies and in most cases that an NUS is recognized by the national government as being the student representative body on a national level.