What are pseudo-classes in CSS?

Pseudo-classes in CSS are used to define special states of an element. This means that they allow developers to apply styles based on the current state of an element rather than just its type or class. For example, a pseudo-class can change the appearance of a link when it's hovered over by the mouse, or highlight a form field when it is in focus.

Using pseudo-classes effectively enhances the interactivity of web pages, as they enable styles to change dynamically in response to user actions without needing to add or remove classes via JavaScript. Pseudo-classes help to create more engaging and user-friendly interfaces by allowing developers to control how elements behave under different circumstances.

The other choices do not accurately encompass the concept of pseudo-classes. They either describe different CSS functionalities or elements that are not related to the dynamic state aspect that defines pseudo-classes.