The limit is 0.50g per liter of blood (it is 0.80g/l in the UK). It is not much : about two glasses of wine for an average person. Your alcohol level might be measured by law and order forces via a breath analizer (usually done should the police/gendarmerie officer suspects you drank, or done randomly).
What happens if you are over the limit?
If you have between 0.5g/l and 0.8 g/l, the fine is 135 euros, to be paid on the spot, loss of six points on your driving licence (if french), and you won't be allowed to take the wheel until you are below 0.5g/l.
If you have more than 0.8g/l : licence confiscated, fine up to 4500 euros and possible jail time up to 2 years, decided by a judge (severity of the sentence depending of course on how drunk you were, behaviour etc), plus 6 points taken off the licence. You might also be banned from driving in France for up to 3 years.
What about drugs?
If you add to that drugs (including canabis) : up to 9000 euros, up to 9 years jail time.
In case of a crash while drunk / high?
If you caused a crash while drunk, up to 30000 euros and up to 2 years jail time in case of serious injuries, up to 150000 euros and up to 10 years jail time in case of fatalities.
Now sure, wines, spirits and beers abound in France. Sure, police forces are not likely to arrest you. But better be safe than sorry.
Enjoy your driving!