According to France’s Minister of National Education, that’s a “college level education institution that recruits its students by tender and ensures high-level graduation”.
Not easily found by its original and classic legal name, they’re also known as Écoles Supérieuresor just École, like the École Nationale d’Administration (ENA) or the École Nationale Supérieure des Techniques Avancées (ENSTA). They are not part of the rest of the University system (and don’t have any equivalent in the US), smaller, but have much more money (they get 30% of the national university budget with only 4% of the students), and are kept apart from the rest of the educational system. That makes getting a job a lot easier after college. You have credibility and can bargain for better salary if you want. After all, you were at one of the 250 Écoles at Paris!
Courses like Law, Medicine, Management and Political are only found in Grande Écoles, because of its importance. They may have other courses also, but these can be studied only there.
Top quality education is expensive and hard to get in. Really hard… The most searched courses can vary a lot in prices! It can be 200 euros or 10 thousand. People start studying really early for the tests at a “Classe Preparatoire” right after high school and has duration of 2 years.
In France, an “ingénieur” is not an “engineer”. It means that he/she studied in one of the “Grandes Écoles” and it is quite prestigious. If it’s one of the best of these schools, it is very likely that he/she is incapable of fixing your car but, after having succeeded in a very competitive academic course, he/she is a manager in a high position.