Originally, a Marquis was a nobleman entrusted with the protection of areas on France's borders called a "marche" or marsh - hence the title. However, as the dukedoms increased in size over the Middle Ages which meant that most of the marquisats (area owned by a Marquis) disappeared into the dukedoms. The title of Marquis is different from that of it's superior Duc since the title of Marquis could be bequeathed to whomever the King saw fit whereas the title of Duc was hereditary; ever since the military importance of the Marquis disappeared it created the perfect option for the King: he could bestow the title to someone he had to reward but without granting too much power.
Louis XIV and Louis XV both had a habit of making their mistresses Marquises; Marquise de Pompadour, Marquise de Montespan, Marquise de Maintenon etc. The title of Marquis is unique in the fact that women can hold the title of "Marquis" in their own right and only really became "Marquise" when she is married to a Marquis. Those who did not have the good fortune to become the King's mistress could buy a title of Marquis far easier than that of Duc.
When the title was introduced - as the last one - it caused widespread outrage among the Comtes that the new-comer should rank higher than they did. The reason for this was that since the Marquises defended the areas that were most under threat from invading armies they were seen as more important than the Comtes who had no such responsibility. Complain as they would, the hierarchy stayed as it was.
Notable French nobles with the title of Marquis/Marquise during the Ancien Regime:
Marquis de Sade
Marquise de Sévigné
Marquise de Pompadour
The Marquis' coronet is very similar to that of a Duc but instead of six solid golden flowers there were three golden flowers and three "flowers" of pearls.
Here are the number of new Marquis-titles created by the three Kings at Versailles:
Louis XIV - 296
Louis XV - 174
Louis XVI - 28