In 2002, the Help America Vote Act required (among other things) that states must maintain a "computerized statewide voter registration list". These lists (henceforth "voterfiles") contain information about every registered voter and their voting history.
When people who have not worked with voterfile data before hear about voterfiles, their first response is almost always "But in my 8th grade civics class, I learned that a critical component of American democracy is the secret ballot! How can states have a list of how you vote?" Voterfiles do NOT include information about how an individual voted. They report whether an individual voted in a specific election.
The exact format and contents of a publicly available voterfile differ from state to state. At a minimum, a file will contain: