Pierogis come in many variations, both sweet and savoury, but what they all have in common (other than being wrapped in dough) is that they're not what a dietitian would recommend for anyone trying to lose weight.
The dumplings are made with a rich dough that contains butter and often sour cream and/or egg. The savoury ones are often filled with potato or cheese, although they can also be stuffed with meat and vegetables. Both the savoury pierogi and the sweet or fruit-filled ones are frequently accompanied by sour cream.
Pierogi go by different names, depending on which country they're from. They're usually attributed to Poland, but I've also tasted them in Czech and Ukrainian restaurants. The dumplings are boiled or fried; often, after boiling, they're pan-fried in butter, which makes the exterior brown and slightly crusty. Savoury pierogi are delicious when served with sautéed onion.