Why did knights become a distinct group and when did they decline?

There were frequent localized wars in Europe during 9th century. Soldiers were not enough. Good cavalry was the need of hour. It led to the growing importance of a new section of people, i.e. the knights. They were linked to the lords. The lord gave the knight a piece of land that was called fief. Land of the fief was cultivated by peasants. In lieu of that, the knight paid his lord a regular fee and also promised to fight for him in war. In this way, they became a distinct group. The fall of feudalism was one of the main reasons of the decline of knights during 15th century.