The federal Help America Vote Act mandates that no voter be turned away without the ability to cast a ballot. Whenever a voter’s registration, identification or residency is in question, the voter is given a ballot nevertheless. 
This voted and sealed ballot is then marked "provisional" and secured separately from other ballots. After Election Day, the voter registrations of provisional ballots are researched. If a registration is deemed valid, the ballot is counted. If deemed invalid, the ballot is not counted.
Provisional ballots may be obtained at a voter service center during their normal hours of operation.