In what time period exactly? France under Phillip I and say, during the 100 years war differs greatly.
Lets take the French Kingdom around the time of the first crusade (Late 11th century). The Kings of France rule a very small, yet rich and prestigeous strip of land surrounding Paris called the Ille de France. France itself is a vast sprawling mass of powerful magnates who were Princes and rulers in their own right. The Counts of Toulouse, The Dukes of Normandy etc. may be considered 'vassals' of the French Kings but this was a very loose definition. More often than not they were fighting wars against the King!
The individual lands of the French Kingdom (I'm actually not even sure if we can talk about the nominal Capetian French Kingdom like this) raised their soldiers along the feudal structure. Each lord, under the fashion of a pyramid, raised troops from his vassals (Or sometimes paid other lords to provide the soldiers) and so and so on until a motley and rough assortment of knights, hobilars (Or horsed seargents) men at arms, and footsoldiers are raised (Such as crossbowmen, archers etc.) to fight for a certain amount of time during the year. Often these lesser lords quarreled amongst themselves and there was sometimes confusion as to what Lord owed what amount of soldiers to whom.
Feudalism certainly isn't a straightforward system of organisation at all. And I'm giving a very bad explanation of it.