Link. At this point, the link is there waiting for the user to click.
Visited. The user has clicked this link before.
Hover. The mouse pointer is hovering over the link.
Active. The link is being clicked (the mouse is pressed on the element and has not been released)
The following are the 4 pseudo-class selectors corresponding to these states (using the a selector and some example declarations):
a:link {color:black;}a:visited {color:gray;}a:hover {text-decoration:none;}a:active {color:red;}