Block Element Modifier (BEM) is a popular methodology for naming CSS classes in a way that promotes clarity and maintainability. It breaks down the user interface into three parts: Block, which represents a standalone component; Element, which is a part of the block that has no standalone meaning; and Modifier, which is a flag that changes the appearance or behavior of a block or element. Using BEM, a class name is structured as `block__element--modifier`. For example, in a button component, the class name `button__icon--large` indicates that the class is for a large icon element within the button block. This structured approach helps developers understand the relationship between components and their styles easily.