Tutorial 4: CSS IDs

IDs are similar to classes, except once a specific id has been declared it cannot be used again within the same (X)HTML file.
I generally use IDs to style the layout elements of a page that will only be needed once, whereas I use classes to style text and such that may be declared multiple times.
The main container for this page is defined by the following.

<div id="container">
Everything within my document is inside this division.
</div>

I have chosen the id selector for the "container" division over a class, because I only need to use it one time within this file.
Then in my CSS file I have the following:

#container{
width: 80%;
margin: auto;
padding: 20px;
border: 1px solid #666;
background: #ffffff;
}

You will notice that the id selector begins with a (#) number sign instead of a (.) period, as the class selector does.