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
Don't let them turn you away from the polls. If your eligibility is questioned, make sure to fill out a provisional ballot. This way, your vote will be counted upon verification of your eligibility. The Help America Vote Act passed in 2002, guaranteed voters provisional ballots if they believed they were eligible to vote. I recently changed my address but my driver's license still has my old address so i became nervous that i'd be turned away, this way i know i still have a right to vote...
Also, ONLY IN CT AND RI, if you are not registered to vote but still want to vote for president, you can. Only Connecticut and Rhode Island permit a resident who is not a registered voter to use the presidential ballot. You'll have to go to your local town hall and ask to vote in the presidential election by casting your vote on the presidential ballot.
ETA: Minnesota has same-day registration as well :)
Also, if you're voting in CT: VOTE "NO" ON QUESTION ONE!!!