box-sizing: content-box (default)
box width: 300 pixels + 10 pixels padding and 1 pixel border on each side = 322 pixels
box-sizing: border-box
box width: 300 pixels, including padding and borders

Code used in this page

.box-sizing {
	width: 300px;
	height: 100px;
	border: 1px solid #0080ff;
	margin-bottom: 20px;
	padding: 10px;
	
	/* Firefox */
	-moz-box-sizing: content-box;
	/* WebKit */
	-webkit-box-sizing: content-box;
	/* Opera 9.5+, Google Chrome */
	box-sizing: content-box;
}

.box-sizing-border-box {
	width: 300px;
	height: 100px;
	border: 1px solid #0080ff;
	margin-bottom: 20px;
	padding: 10px;
	
	/* Firefox */
	-moz-box-sizing: border-box;
	/* WebKit */
	-webkit-box-sizing: border-box;
	/* Opera 9.5+, Google Chrome */
	box-sizing: border-box;
}