I realize that is a very ambitious title. Books have been written and careers made on this topic. My only intention is to share with you, my non-French audience, what I have observed. Disclaimer: fashion is not my thing (yes thank you, I am Captain Obvious).
Alright, first off, there is one stereotypical French piece of clothing. This shirt is the French casual go-to uniform. You know the one--small blue and larger white horizontal stripes, usually 3/4 or long sleeved (although I've seen all sleeve lengths) and at least slightly boatnecked. I kid you not, every time I go into town I see at least one person wearing a variant of it. This plus a Longchamp bag will fool almost anyone into thinking you're French. Until, of course, you open your mouth.
Then it can get weird. There are all kinds of variations of weird. There's the lady coming out of the mall at 2pm on Tuesday wearing a tiny, fancy red dress and sky-high heels. I would never have the guts to leave the house in that (again, I'm not very gutsy in clothing, but she was really quite overdressed for the mall). But there she was, pushing a stroller, smoking a cigarette, and walking beside hubby, much more situationally appropriate in a tshirt and jeans.
Another variant of weird is the patterners. These are my favorite. I noticed once or twice last semester a few people doing some odd pattern mixing, but I think it was usually girls and for some reason they could pull it off. Now that it's really hot here though, the guys have gotten in on it and it's hilarious. Red and black striped shirt with green and brown plaid shorts? Not just for the laundromat here. Nope, that's going-out attire. Let's be seen, baby! Floral and plaid? NICE! I do not understand. These are some true hurt-your-eyes mixes here. And these guys are often walking around with nicely dressed girls! I've suggested the bf change outfits for infractions not even close to that bad... Maybe it's actually some kind of high fashion statement that is just way above my head, like actually red and black stripes compliment green and brown plaid in a way I will never comprehend. How gauche of me. Le sigh. I think I'll just stick with what I can handle. I wouldn't want to break any pattern-mixing rules I was unaware of!
Final point about French fashion. French scientists? They dress like US scientists. You get a healthy mix of people wearing shirts their older brother grew out of in 1987, those dressed functionally yet socially acceptably (if sometimes a bit scruffy), and a few who manage the mainstream French style. Funny thing though? Those can be the same person on consecutive days... keeps you on your toes!