Primaries work in different ways and are classified by category.
State Primary Categories
Open: Voters may vote in primaries of a party of their choice, the choice to be made at the voting booth. In other words, you can choose which party's primary to vote in, regardless of your own party (or lack of) affiliation. You CANNOT, however, vote in more than one party primary - you must choose.
Closed: Voters must declare party affiliation in advance of the primary election and can only vote in that party's primary. To participate in your party's closed primary election, you must identify party affiliation when you register to vote.
Semi-closed: Voting is restricted to voters who have already declared their affiliation to the party in advance and new and/or independent (non-affiliated voters) who choose which primary to vote in on election day. To participate in your party's semi-closed primary election, you must identify party affiliation when you register to vote.
Semi-open: Voters can choose which party's primary to vote in on election day, regardless of their party affiliation, but must do so publicly.