The Pennsylvania Voter Registration Application has a field to enter a choice of political party. The options are: Democratic, Republican, Green, Libertarian, None (No Affiliation) or Other.
To cast a ballot in a primary election, a voter must be registered as a Democrat or Republican.
To vote in a municipal or general election, a party affiliation is not required.
When Elections are Held provides a guide to the types of elections and when they take place.
To learn about the four biggest political parties in the U. S., their websites are a good starting place: