The style attribute is the most important common attribute as it determines how your site looks and acts.
In old standards of HTML, styles would normally be attributed by simply listing individual style elements:
Code:
<div width="300" bgcolor="black">
In XHTML, the standard is to use the style attribute:
Code:
<div style="width: 300px; background-color: #000000;">
So basically, the style attribute encapsulates all the "customization data" for a particular element.