Pamela is an epistolary novel written in the form of letters but through Pamela letters, we can also read her psychological introspection so the novel could also be considered a psychological novel.
In Richardson's period the work Pamela represented a sort of tutorial of good manners for the middle class who enjoyed reading about characters and events in which they could recognize themselves.
As a matter of fact, Pamela is from the low-class: she is a young servant in a quite rich family. At first she works for the mother of so-called Mr B, but when his mother dies, she starts working for him. She never gives up, and during this period the only confort she has is writing letters to her parents and inform them about events. When she wants to go and visit her parents, Mr. B provides her a coach, takes her in his estate and tries to make avances, but she preserves her virginity. In the end he falls in love with her and proposes to her, so they get married.
The Richardson choice of a servant as main character was a quite revolutionary so Pamela became a sort of model for all the people.
Regarding the novel there are two points of view:
1) the propamelists thought that Pamela's marriage was the right reward for her good behaviour.
2) the antipamelist said that she behaved like that on purpose in order to convince him to marry her and thought that she was a social climber.
The novel was so successful in that period that a phenomenon regarding the character of Pamela developed and a sort of gadgets started being sold. In contemporary world it would be considered a best-seller.
Pamela was also considered suitable to be staged, because there are a lot of witty dialogues.