An exit poll is one that is conducted after voters have placed their ballots at their respective polling locations. Exit polls will actually ask individuals whom the person actually voted for, which differs from other types of polls which are based on an opinion or how a person plans on voting.
The ultimate purpose for exit polls is simply to provide for a more clear prediction as to which candidate may have one a particular election. Most election results will often take a certain period of time before the actual results are produced and made public, and thus, an exit poll can be one method in providing for an early indication as to how the election concluded.
Exit polls also allow for certain demographic data to be collected in terms of the voters questions, often times taking into considerations aspects such as sex, age, party affiliation, and ethnic background to provide for an understanding in certain tendencies or inclinations as to why groups of people voted in a particular way. However, even though exit polls can be used as a way to provide preliminary and unofficial results of an election, there are certain controversial issues that will arise.
One particular aspect that is under scrutiny is that major media outlets may use exit polls as way to publicly announce that a particular candidate has one the election before all the votes are accounted for. In the United States, the most recent and notorious account of such a situation revolves around the 2000 United States Presidential Election and the debacle that occurred with the state of Florida. Many believed that the media release results before all of the polls were closed and the votes were officially accounted for.