" Bac " is an abbreviation for " baccalauréat ". And the word " baccalauréat " has the same root with spanish word " bachilierato ". But unlike the spanish term, le bac doesn't refer to a type of studies in high school, but only to the final exam. It is the most important exam in once scolar coreer. It sanctions12 years of studies. Getting the bac is the only way to finish up the high scool. It is also a basical requirement to follow post secondary studies. The baccalauréat is considered as the first uniniversitarian grade. The second being the licence and the thrid and finall being the doctor grade. I hope it helped you to understand it.

John is right. But the Bac has also become a sort of initiation, which marks the first "true" exam and the entrance in the (more or less) autonomous life. It's now a symbol and something somewhat important in the young's life. The fun side is this exam is also strictly useless in itself, because of the fact that you have to do other studies afterwards (or in place to, for some "manual" jobs) to hope to get an interesting job (except for people who manage to get a job without diploma). So a lot of people think the Bac is now just archaic and that we should mark differently the end of the school years and beginning of the university. But this is a symbol...

I must add, this exam is very old and kind of tradition. French people are attached to it. It was created by Napoleon, and generations of french have taken it. It is part of the french culture. Every year people share the given subjects of philosophy in which the students have to discuss a simple philosophical sentence during four hours ( for example, " Am I what my past made of me ? " - you have 4 hours-, or " Is truth an objective for art ? - you have 4 hours-). And when the exams are finish and the papers corrected, the high school students and all their families are very stressed and go to check if they succed or not. If yes, they celebrate it.

I think it's the standard national exam they have at the end of high school. I think it also acts as a university entrance exam. I looked at the math test they have as part of it. It's pretty hard.