hover (:hover)

The :hover CSS pseudo-class matches when the user designates an element with a pointing device, but does not necessarily activate it. This style may be overridden by any other link-related pseudo-classes, that is :link, :visited, and :active, appearing in subsequent rules. In order to style appropriately links, you need to put the :hover rule after the :link and :visited rules but before the :active one, as defined by the LVHA-order: :link — :visited — :hover — :active.
 
Visual user agents, like Firefox, Internet Explorer, Safari, Opera or Chrome, apply the associated style when the cursor (mouse pointer) hovers over an element.

Example

<!DOCTYPE html>
<html>
    <head>
        <style>
            a:hover {
                background-color: #F60;
                color: #FFF;
            }
        </style>
    </head>
    <body>
        <a href="#">Hover Mouse Here</a>
    </body>
</html>