Understanding the importance of class in eighteenth- and nineteenth-century Britain is essential to understanding Wuthering Heights. Generally, at the time, people were born into a class and stayed there: if your parents were rich and respected (like Edgar's), you would be, too; if your parents were servants (like Nelly Dean's), you probably would be too. Social mobility—the idea that you can change your class status (usually for the better)—was not commonplace.
In Brontë's novel, however, class distinctions are constantly changing, much to the confusion of the characters. There are two primary examples of this: Heathcliff and Hareton. Because no one knows anything about Heathcliff's background, they all treat him differently. Mr. Earnshaw adopts him and treats him like a son, but the snobby Lintons refuse to socialize with him. When he disappears for a few years and comes back rich, the characters struggle even more over how to approach him—he now has money and land, but many of them still consider him a farm boy. Likewise, Hareton has a hard time gaining respect. The son of Hindley, Hareton should be the heir to Wuthering Heights. With land and standing, he ought to be a gentleman. However, Heathcliff refuses to educate him, and everyone else mostly ignores him, so his manners (a very important indicator of class status) are rough and gruff. Only when young Cathy helps educate him does he achieve the class standing to which he was born.