"The Courtier disdaineth the citizen; The citizen the countryman; the shoemaker the cobbler. But unfortunate is the man who does not have anyone he can look down upon." ~ Tomas Nash, 1593
In theory, there are but two classes of people: Nobles and Commoners. In practice, there are a huge number or gradations of both classes. These gradations are thought of as parts of a Great Chain of Being, which extends from God down to the lowest forms of life, and even to the trees and stones of the earth. This Great Chain, first described by St. Thomas Aquinas, is what holds the world together. The Great Chain is as follows:
For Catholics, the Pope is at the same level or above the King.
Speaking of clergy, the Church hierarchy is actually separate from the secular hierarchy. I have inserted churchmen into the Chain at the best approximation of their ranks.
In terms of deference, personal threat matters. In other words, you might bow to a pirate even if you technically out-rank him, because he is armed and you are not!
Office also makes a difference. The King's most trusted advisor gains deference greater than that to which his rank entitles him.
The term "Yeoman" is used here to distinguish a farmer who owns his own fields from one who is merely a tennant on someone else's fields. The term does have several other meanings in other contexts. Be thou not confused!
Children have, in general, a rank one or two beneath their parents while they remain minors.
There are as many gradations among the non-human orders as among people. A complete list would simply be too long to reproduce!
For more on how the links of the Great Chain interact, including some good practical advice, check out this article.