Tennessee uses an open primary system, so voters do not have to register with a political party before Election Day. When you go to vote in a primary, you choose either the Republican or Democratic ballot and vote only in that party’s races. The candidates who receive the most votes in each party’s primary advance to the general election, where they compete against candidates from the other party.
The winners of each party's primary go on to the general election in November.
See democrats.org, tndp.org, gop.com, and tngop.org for more information on each party.