The ID is used for one HTML tag.
For example:
<style type=”text/css”>
<!– #aaa {color:#fff} –>
</style>
<p id=”aaa”>xxx</p>
One CLASS is used for many HTML tags generally.
For example:
<style type=”text/css”>
<!– .aaa {color:#fff} –>
</style>
<p>xxx</p>
<table>xxx</tabe>