what kind of plant i this?

the plant was planted by my former korea home owner. The root is trying to creep everywhere. I trie the leave and boiled it in water, very bitter. Does anyone know what it is and what\'s the usage of this plant? if it is eatable, how to cook it?thanks!

http://tinypic.com/view.php?pic=259a49h&s=5



http://tinypic.com/view.php?pic=equk1x&s=5