Most basic example (live example here):
CSS:
#wrapper {
width: 500px;
margin: 0 auto;
}
HTML:
<body>
<div id="wrapper">
Piece of text inside a 500px width div centered on the page
</div>
</body>
How the principle works:
Create your wrapper and assign it a certain width. Then apply an automatic horizontal margin to it by using margin: 0 auto; or margin-left: auto; margin-right: auto;. The automatic margins make sure your element is centered.