Can someone tell me what the middle class is during the medieval times and how it came to exist?

The rise of the middle class came from two different groups of people, upper class families were one becuase of the amount of money it took to become a succesful merchant / trader. The other group was merchants from the lower class who were often looked down upon but because of their succes in their respective guilds they were brought together. The middle class grew in importance due to the fact that there was a significant growth in the number of towns. The rise of the middle class created a merchant class, responsible for the growth of cities, increased trade, and lifted people from the lower class.