PIE stands for Progressive Internet Explorer. It is an IE attached behavior which, when applied to an element, allows IE to recognize and display a number of CSS3 properties. Consider, if you will, the following CSS:
PIE currently adds full or partial support to IE 6 through 8 for the following CSS3 features
- border-radius
- box-shadow
- border-image
- multiple background images
- linear-gradient as background image
http://css3pie.com/about/