What Is a Write-in Candidate?

A write-in candidate is a candidate in an election whose name does not appear on the ballot, but for whom voters may vote nonetheless by writing in the person's name. Some states and local jurisdictions allow a voter to affix a sticker with a write-in candidate's name on it to the ballot in lieu of actually writing in the candidate's name. Write-in candidates rarely win, and votes are often cast for ineligible people or fictional characters. Many jurisdictions require write-in candidates to be registered as candidates before the election.[citation needed] This is usually mandatory in elections with large pools of potential office-holders, as there may be multiple people with the name that is written in.

Write-in candidates are a holdover from the time when ballot papers were blank, and had no names printed on them at all. Gradually, the ballots were arranged to have all the names of the candidates printed on them, with a "write-in" provision for latecomers.

Many states and municipalities allow for write-in votes in a partisan primary where no candidate is listed on the ballot to have the same functional effect as nominating petitions: for example, if there are no Reform Party members on the ballot for state general assembly and a candidate receives more than 200 write-in votes when the primary election is held (or the other number of signatures that were required for ballot access), the candidate will be placed on the ballot on that ballot line for the general election. In most places, this provision is in place for non-partisan elections as well.

source: http://en.wikipedia.org/wiki/Write-in_candidate