<body onkeypress="var e = (event ? event : window.event); return (e.keyCode ? e.keyCode : e.charCode ? e.charCode : e.which) != 13;" >
Essentially that is saying grab the event, determine which key was pressed, and return false if it was the enter key. Returning false suppresses the form from auto-submitting.
Suppose you have a really super useful event handler like the following:
<input type='text' onkeypress='alert("Wow this sure is annoying");' />
The alert would still be shown even though you have the enter key-eating code at the body level thanks to the bubbling precedence of event handling. In essence, the onkeypress event handler is called for the textbox before the event handler code for the body is called. (If you return false from the textbox, it would never bubble up to the body's event handler)
This has been tested on the various current versions of IE (6,7,8), Firefox (2,3), and Safari (3).