According to Wikipedia, a provisional ballot is used to record a vote when there is some question in regards to a given voter's eligibility. A provisional ballot would be cast when:
The voter refuses to show a photo ID (in regions that require one). The voter's name does not appear on the electoral roll for the given precinct. The voter's registration contains inaccurate or out-dated information such as the wrong address or a misspelled name. The voter's ballot has already been recorded. A provisional ballot is counted contingent upon the verification of that voter's eligibility.