What is the different of class and id in CSS?

Please see an example first:

In my CSS class of my blog I defined a class name
section-w {
border-top:none;
padding:14px;
margin-bottom:11px;
}
and a class name
#main_left {
width:551px;
float:left;
line-height:18px;
display:inline;
border-top:none;
border-right:solid 1px#DEDEDE;
}

and for 'section-w' as :
<div class="section-w" id="categories">
....
</div>
and
<div class="section-w" id="adsense-sidebar">
....
</div>
and I applied it in HTML code 'main_left' for as:

<div id="main_left">
<table width="551" border="0" cellpadding="0" cellspacing="0">
<tbody><tr><td width="551">
....
</td></tr></tbody></table>
</div>

So you can see the different between class and id is:

Class can be used for many times in HTML code, but id can be used one time.