http://en.wikipedia.org/wiki/Pierogi
They're really good. My friend in HS invited me up one day when she and her sis and mom and grandma were following an old family tradition of making hundreds of them. The Grandma was straight from Croatia or something, so they were traditional. And quite good.
Now I just buy frozen Mrs T's when I get a hankering. Filled with potato and cheddar cheese.