Below are the various ways you can use CSS to style links.
a:link {color: #009900;}
a:visited {color: #999999;}
a:hover {color: #333333;}
a:focus {color: #333333;}
a:active {color: #009900;}
Now lets take a look at what each one of the above link styles actually does.
a:link {color: #009900;}
The first on the list sets the color of a link when no event is occuring.
a:visited {color: #999999;}
The second sets the color a link changes to, when the user has already visited that url.
a:hover {color: #333333;}
The third sets the color a link changes to as the user places their mouse pointer over the link.
a:focus {color: #333333;}
The fourth is 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;}
The fifth on the list sets the color a link changes to as it is pressed.
You must declare the a:link and a:visited before you declare a:hover.
Furthermore, you must declare a:hover before you can declare a:active.
Furthermore, you must declare a:hover before you can declare a:active.