Pseudo classes are classes used by Web browsers to differentiate statuses of a HTML tag. CSS definitions can use pseudo classes as selectors with a leading colon like (:visited). There are 3 pseudo classes on the <A> tag:

► A:link - A hyper link that has not been visited.

► A:visited - A hyper link that has been visited before.

► A:active - A hyper link that is currently under the mouse pointer.

If you want to the alter the default style sheets of the browser, you could define something like this:

/* show un-visited links in blue */

A:link {color: blue}

/* show visited links in yellow */

A:visited {color: yellow}

/* change background color gray when mouse over links */

A:active {background-color: gray}