Realism
Critics tend to label Stendhal as one of the first writers of "psychological novels," meaning that he writes novels that delve deeply into the desires and motives of his characters. You can think of this type of literature as being part of the genre of Realism.
Realism aims to offer a realistic portrayal of the world, especially its different social classes. Julien Sorel comes from a lower class background and struggles to lift himself into the highest realms of French society. The question is whether he's willing to compromise on his personal values to make this happen (and the answer is a resounding yes) and that's where the psychology part comes in.