This example demonstrates Pseudo Classes in CSS.
A pseudo class isn't really a style class at all, but refers to a specific state of an HTML element that can be styled.
The rest of the explanation for this example is in the source code.
Chicago Cubs Page
Chicago Bulls Page
Chicago Bears Page
This is some text with a span container in the middle to show a span with a hover state.
This is a div block with a hover state. Although, doing this to a block is generally not useful in practice, this shows that the hover state can dramatically change an HTML element.