What are pseudo-classes in CSS?

In CSS, pseudo-classes are used to define a state of an element based on its position, relationship with other elements, or user interaction. 

They are denoted by a colon (:) followed by the pseudo-class name.

Examples of pseudo-classes:

1. :hover - applied when an element is hovered over by a user

2. :active - applied when an element is being clicked or activated

3. :focus - applied when an element receives focus (e.g., when a user clicks on a form input)

4. :visited - applied to links that have been visited by the user

5. :first-child - applied to the first child element of a parent element

6. :last-child - applied to the last child element of a parent element

7. :nth-child(n) - applied to the nth child element of a parent element

8. :disabled - applied to elements that are disabled

9. :enabled - applied to elements that are enabled

10. :checked - applied to checkboxes and radio buttons that are checked

Pseudo-classes allow you to apply styles to elements based on their state or position, without needing to add additional classes or attributes to the HTML.

Side note: In programming languages, pseudo-classes can also refer to a mechanism to simulate the existence of classes or inheritance in languages that do not support these features.