The French concept of nobility was very different from the English one. Whereas, in England, only a peerage bestows nobility on the holder, in France, nobility was a quality, a legal characteristic of the individual, which was held or acquired in specified ways, and which conferred specified rights and privileges. The manners of acquiring nobility being specific, French nobility isn’t the same as the English gentry either, which has no legal definition or status.
Nobility was usually a hereditary characteristic, but some forms of nobility could not be transmitted. When it was hereditary, nobility usually came from the father, but sometimes a higher percentage of noble blood might be required (counted in number of “quartiers”) or that the family be noble for a certain number of generations. A nobleman marrying a commoner did not lose his nobility, but a noblewoman who married a commoner lost it, as long as she was married to the commoner.
Nobility was an important legal concept, in particular because of the privileges attached to it. Taxes were originally levied to help the sovereign in times of war; and since nobles were expected to provide help in kind, by fighting for their sovereign, they were usually exempted from taxes. This privilege lost its rationale after the end of feudalism and nobility had nothing to do with military activity, but it survived for the older forms of taxation until 1789 (more recent taxes, levied in the 17th and 18th centuries, allowed for weaker or no exemption for nobles).
A number of offices and positions in civil and military administrations were reserved for nobles, notably all commissions as officers in the army. This privilege created a significant obstacle to social mobility and to the emergence of new talents in the French state. It remained very real until 1789.