Unlike architecture or fashion, one cultural difference that takes a lot of close observation (and inadvertent social gaffes) to master is etiquette. Especially in France, the "rgles de politesse" (literally "politeness rules") are of the utmost importance.
After more than five months of living here, I feel as though I have only just decoded them enough to function in society without offending anyone too much. Though of course I'm still learning - yesterday my host mom told me I shouldn't sit pretzel-style!
The most basic "rgle de politesse" is knowing when to use "vous" (formal "you") and when to use "tu" (informal "you"). You should always address strangers, service workers (e.g., waiters, cashiers), and your elders with "vous"; and peers, children, and family members with "tu." When in doubt, you should always use "vous," since it never hurts to be too polite.
My host family has explained to me that the difference between "vous" and "tu" also has significant implications in the workplace. Using "vous" keeps a certain distance between coworkers and a boss, for instance, whereas "tu" results in a much more casual environment, for better or for worse. Personally, I've found that I quite like that using "vous" automatically adds an extra level of respect.
The second most essential "rgle de politesse" has to do with greeting others. Saying "bonjour" or "bonsoir" (depending on the time of day) upon entering a shop, waiting room, etc. is expected. Children and teenagers are expected to be the first to say "bonjour" when greeting their elders. The same idea of greeting goes for friends at school; the first time you see anyone you're on friendly terms with during the day, you greet them. Girls greet each other and boys with "la bise" (double cheek air-kiss) and boys shake hands (unless they are very close friends, in which case they also do "la bise").
It almost goes without saying that, in France, many of the "rgles de politesse" revolve around mealtimes.
At more formal dinners, guests are served before their hosts, generally starting with the oldest woman and working down to the youngest man.
During these meals, the female head of the family is expected to serve the food and clear the places afterward (as meals are often several courses long). The male head of the family takes care of the wine and any daughters or girls of the family help with the clean-up.
Even in the "cantine" (lunchroom) and other informal settings, there are a few guidelines. Someone usually wishes the others "bon appetit" before starting, the "entre-plat-dessert" (appetizer-main dish-dessert) order of eating is strictly observed, and everyone waits until everyone else is served and ready before starting to eat.
Being familiar with a country's etiquette is almost as important as knowing its language, and leads to a much smoother experience once understood. So, au revoir for now!