What is the CSS feature - "box-sizing"?

For W3C, width and height values contain only content area. Padding and border values are not contained by these values. Most browsers use this standart.

However, IE6 and previous versions of IE6 accept such a rule that width and height values contain not only content area, but also padding and border values.

Because of this, W3C creates a feature in css to give a chance to user for selection of box-sizing type;

Structure : box-sizing: <value>
Values : content-box | border-box
Default value: content-box
Applicable elements: All elements
Inheritence: No

References: 
1) https://css-tricks.com/box-sizing/
2) http://fatihhayrioglu.com/css3-box-sizing-ozelligi/
3) http://www.w3schools.com/cssref/css3_pr_box-sizing.asp