There are actually two different agrégations:
In addition to those teaching in normal sections of lycées (the vast majority), some agrégés teach in the preparatory classes to the grandes écoles. Finally some agrégés teach in normal universities, but do not, nominally, do scientific research as normal university academics do ; these positions are known as PRAG. Some similar but temporary positions (agrégé préparateur, AGPR), including research, exists in the écoles normales supérieures, though these are obviously very few and very hard to obtain.
The agrégation is normally open only to holders of a 4-year college education (formerly, maîtrise) or above. Its preparation often takes a full additional year at the university, for the so-called external agrégation. There also exists an internal agrégation for professeurs certifiés, though it lacks the prestige of the external one. The following discusses the external one.
The exam generally consists in a written part (admissibility) where most candidates are eliminated, followed by an oral part (admission) where the candidate must demonstrate his ability to prepare and give lessons on about any topic within the scope of his discipline.
In most disciplines, the lessons expected extend well above the secondary education level; the candidate may, in effect, have to present a lesson appropriate for the 2nd, 3rd or even 4th years of specialized studies at a university. One reason for that is that the agrégés should be able to teach in special undergraduate sections of high schools known as preparatory classes to the grandes écoles, where the level is above the normal level of 1st or 2nd year college education – though the vast majority of agrégés teach in ordinary secondary education.
The agrégation is also used as a kind of national ranking system for students, giving a fair comparison between students of different universities. This is especially true in the humanities, where the agrégation is highly selective and serves as a test on the culture of the candidate. The students of the écoles normales supérieures often give up an entire year of their adult life to prepare for any potential question.
Some sociologists like Pierre Bourdieu have argued that this exam measures a candidate's social connections as much their ability to present a lesson — consider, for example, that a candidate for a Law professorship may be helped by first-class attorneys if he happens to know them.