PO, Citizen's Platform, the center-right liberal (in the European economic sense) ruling party. Very pro-European, pro-German.
PiS, Law and Justice, mainstream right wing nationalist party, very Catholic, pro-Europe economically but fighting culture wars over in vitro, gay marriage and so on.
PSL, Polish People's Party, agrarian party with roots stretching back to before WWII. Represents rural interests.
SLD, Democratic Leftist Union, the post-Communist left that discredited itself horribly when last in power ten years ago (through corruption scandals) and is still in the political wilderness. Pro-EU, very much a modern European left party in terms of platform but dominated by faces from the bad old days.