A lot of sites don't span the full screen length (i.e. fixed width sites), so webmasters use a "wrapper" to set the appropriate width and act as a full screen would. Something I do from time to time:
width: 760px; }
<div id="wrapper">main html here</div>
Not a perfect example but it gets the point across. It's probably bad practice to include all the page HTML in between <center> tags.