The default display style of a <div> tag is BLOCK.
The best way to horizontally center an element is to set the left-margin and right-margin to auto or use the margin: 0 auto declaration.
Padding is the space between an element and it's border ("insidey" or "indoorsy"), while margin is the space between that border and an adjacent element ("outsidey" or "outdoorsy").
Fixed positioning places an element in relation to the browser window, while absolute positioning places an element in relation to it's container.
A clearfix will auto-size a floating element's container to match the floating element itself. Otherwise, the element can flow outside it's container in wonky ways.