in MI (which has a non-insignificant polish immigrant population), a paczki (pronounced punch-key or poonch-key) is a large, high calorie, donut like, filled pastry. Jelly is common, but there are also cream and custard filled varieties. The “traditional” style is prune filling, tho. Generally only eaten for Paczki Day, another name for Fat Tuesday prior to Lent.
The most traditional flavors are plain, plum butter (sometimes translated as “prune.” It’s made of stewed plums with that have been cooked down into a jam; it is not made from dried plums) and rosehips. Pączki do not have to be filled, but most people associate pączki with some sort of filling. It’s made from a very rich, brioche/challah-like dough, so the pastry itself is not like what you get from a jelly donut at Dunkin Donuts. It’s heavier and richer and more bread-like. Also, traditionally they have a modest amount of filling, like maybe a tablespoon at most, but these days a lot of the commercial pączki are (IMHO) way overstuffed with filling.