From Academic Kids
An exit poll is a poll of voters taken immediately after they have exited the polling stations. Unlike an opinion poll, which asks who the voter plans to vote for or some similar formulation, an exit poll asks who the voter actually voted for. Pollsters - usually private companies working for newspapers or broadcasters - conduct exit polls to gain an early indication as to how an election has turned out, as the actual result may take hours to count (as in UK General Elections).
Exit polls have historically and throughout the world been used as "parallel vote tabulation", as a check against and rough indicator of the degree of fraud in an election. Some examples of this include the Venezuelan recall referendum, 2004, the Ukrainian presidential election, 2004, and the 2004 U.S. presidential election controversy.
The word "indication" is key as, like all opinion polls, exit polls do by nature include a margin of error. A famous example of exit poll error occurred in the 1992 UK General Election, when two exit polls predicted a hung Parliament. In the event, the Conservative Party Government under John Major held their position with a reduced majority.
In contrast, very few people in Britain could not have predicted Tony Blair's Labour victory in 1997; in that event only the scale of the victory remained unpredictable.
Widespread criticism of exit polling has occurred in cases, especially in the United States of America, where exit-poll results have appeared and/or have provided a basis for projecting winners before all real polls have closed, thereby possibly influencing election results. Leaks of exit poll figures for the 2004 presidential election, mainly via the Internet, appeared to indicate a victory for John Kerry. The discrepancies between the exit poll data and the vote count that where outside of the margin of error, coupled with irregularities in the election which seem to explain the discrepancies and what many perceive as evasive tactics by the polling companies, have shed doubt on the legitimacy of that election among political activists and government officials. (See 2004_U.S._presidential_election_controversy, exit polls for more detail.)
Some countries, such as the United Kingdom, punish the publication of exit poll figures before the polling stations have closed as a criminal offence.