Defines how the width and height of the element are calculated: whether they include the padding and borders or not.
The width and height of the element only apply to the content of the element.
For example, this element has
The full width is 24px + 60px + 200px = 284px.
The content has the defined width. The box accomodates for those dimensions.
The width and height of the element apply to all parts of the element: the content, the padding and the borders.
The full width is 200px, no matter what.
The box has the defined width. The content accomodates for those dimensions, and ends up being 200px - 60px - 24px = 116px.