why when what how to style this in css code example

Example 1: css how to style a

/*a normal, unvisited link*/
a:link{
	color: green;
}
/*a link the user has visited*/
a:visited{
	color: purple;
}
/*a link when the user mouses over it*/
a:hover{
	color: yellow;
}
/*a link the moment it is clicked*/
a:active{
	color: brown;
}

Example 2: how to use css

<link rel="stylesheet" href="stylesheet.css">
<div class="mydiv"></div>
<style>
.mydiv {
background-color: /*Color*/;
color: /*Another color*/;
}
</style>