What was the difference between a knight and a vassal?

The knights were originally the heavy cavalry. The Word knight came to be a title, usually given by a monarch, and was regarded as the lowest level of the nobility.

A vassal was a person who had a relationship of mutual obligation with a lord. The vassal sWord loyalty and obedience, and the lord granted land and protection in exchange.

A vassal could be a knight, and a knight could be a vassal, but there was nothing saying they had to be.