From what I can fathom these are the ways you can use CSS to style links.
a:link {color: #009900;}
sets the color of a link when no event is occurring
a:visited {color: #999999;}
sets the color a link changes to, when the user has already visited that URL
a:hover {color: #333333;}
sets the color a link changes to as the user places their mouse pointer over the link
a:focus {color: #333333;}
primarilly for the same purpose as the last one, but this one is for users that are not using a mouse and are tabbing through the links via there keyboards tab key, it sets the color a link changes to as the user tabs through the links
a:active {color: #009900;}
sets the color a link changes to as it is pressed
However, the Volunteers who help with CSS will give you a hand once this is moved to the CSS forum.