To a browser, all elements contained in a document are considered boxes of content (text, images, etc). Each box has the following attributes:
- padding (surrounding the content)
- border (after the padding, background-color property stops here)
- margin (after the border, before the other elements)
The most common box model problem: in IE 5, IE 6 and IE 7, and in quirks mode, the padding shrinks the content instead of expanding the size of the box, as in the other browsers.
You have the choice to pick what box model applies to elements. The attribute is: box-sizing, and the possible choices are: content-box (default, the padding / border pushes out of the content), or border-box (the padding / border pushes inwards, toward the content instead).
It is recommended you set that as part of your reset, so you have all browsers following the same convention. Then again, that would be like saying IE6 was right all along…
Note that this does not apply to margins! Those will still push stuff out, spawning right out of the boxes borders.