Firstly, gendarmes are in the military. They have different rules as to the use of firearms for example. This also implies that most gendarmes live in barracks, and that citizens can volonteer. Also, if an investigation has to do with the military, the gendarmes would be in charge. Secondly, they don't work in the same areas. Gendarmes take care of the countryside, villages and some small towns (there's a maximum number of inhabitants). On the other hand, in big cities, you'll only see poilicers (apart from the soldiers due to the current vigipirate level). Depending on where you are, they both give fines to drivers ;) Thirdly, they have their own elite force (like the SWAT in America). GIGN for the gendarmes, RAID for the policemen. Finally, the Garde Républicaine (those protecting the President and the Ministers) belongs to the Gendarmerie. There are probably other differences that I don't know of, but most of the time, if you want to press charges, you go just go to the local gendermerie or commissariat depending on where you live. There isn't much of a difference in everyday life.

to summarise. The police works in the cities, while the gendarmes work rather in the countryside.