A pseudo-class in CSS is a keyword added to a selector that specifies a special state of the selected element(s) that can’t be targeted using simple selectors. Pseudo-classes allow you to style elements based on their interaction state or position in the document. There are several kinds of pseudo-classes, including but not limited to:
1. :hover: Selects an element when a user hovers over it.
2. :active: Selects an element when it is being activated by a user (e.g., clicked on).
3. :first-child: Selects the first child of its parent element.
4. :nth-child(n): Selects elements based on their position in a parent element (e.g., every third element).
5. :not(selector): Selects elements that do not match the given selector.