那位大侠可以教我一下伪类的用法

<style>
a.demo{background:#F0EBD6;padding:8px;width:240px;text-decoration:none;color:#000000;}
a.demhover{background:#FFC080;color:#000000;}
a.demactive{background:#CCCFFF;color:#000000;}
a.demvisited{background:#FF3300;color:#000000;}
#idCodeDiv{width:100%;padding:4px;font-family:verdana,tahoma;margin:12px 0px 0px 0px;background-color:#EEEEEE;font-weight:bold;}
</style>


<a class=demo href="none.html" onclick="return false;">With a hush upon the marsh , the afternoon slid . Into the late, immobile period, The sun glazing the reeds and coating the limestone quarry as I, in repose With the marsh, bathed in solitude and splendor.</a>
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<br>
<div id=idCodeDiv>a :link{background:#F0EBD6;padding:8px;width:240px;text-decoration:none;color:#000000;}<br>a :hover{background:#FFC080;color:#000000;}<br>a :active{background:#CCCFFF;color:#000000;}<br>a :visited{background:#FF3300;color:#00000</div>

看不明白，谁能解释一下

就比如说这一句吧
p{first-letter:300%;}
意思是说p这个段落的首写字母比其它的大三倍
这个first-letter就叫伪类了