From the Seattle Times:

Provisional ballots are used when voters cast ballots outside their regular polling place or when there are uncertainties about someone's eligibility to vote. They also include voters who requested absentee ballots but didn't receive them.

Democrats are focusing on provisional and absentee voters in King County, where most votes counted so far have gone to Gregoire.

"We are calling the folks whose ballots were rejected and then we're going to their homes trying to help them to get their votes to count," Berendt said.

Ryan Bianchi, communications assistant for the Gregoire campaign, said the canvassers knock on doors and ask if the person is a Gregoire voter. "If they say no, we just tell them to have a nice day," he said. If they said yes, then they continued with the process.