In the CSS, a class selector is a name preceded by a full stop (.) and an ID selector is a name preceded by a hash character (#).
So the CSS might look something like:
Code:
#top {
	background-color: #ccc;
	padding: 1em
}
.intro {
	color: red;
	font-weight: bold;
}
The HTML refers to the CSS by using the attributes id and class. It could look something like this:
Code:
<div id="top">
 <h1>Chocolate curry</h1>
 <p class="intro">
This is my recipe for making curry purely with chocolate</p>  
<p class="intro">Mmm mm mmmmm</p>  
</div>
The difference between an ID and a class is that an ID can be used to identify one element, whereas a class can be used to identify more than one.
You can also apply a selector to a specific HTML element by simply stating the HTML selector first, so p.jam { whatever } will only be applied to paragraph elements that have the class 'jam'.