
A div (short for division) is a way of defining structural components of your own choosing. In conjunction with the identifiers, "class" and "id" they provide a key structural component of CSS.
Although CSS allows positioning text and other elements by means of floats, putting the content in “logical” order ensures that the document makes sense in browsers that do not support CSS. As a corollary, use document flow when possible.
EMs, percentages Heights are rarely specified-containers expand to encompass content. Use absolute units when the dimensions of the output medium are known (print).
When "floating" elements use the "clear" property on adjacent elements to position. Elements that have a horizontal or vertical relationship may be wrapped in a div.