From Academic Kids
Vote swapping is the method where a voter in one district agrees to vote tactically for a less-preferred candidate or party who has a greater chance of winning in their district, in exchange for a voter from another district voting tactically for the candidate the first voter prefers, because that candidate has a greater possibility of winning in that district.
This occurs informally (i.e., without binding contracts) but sometimes with great sophistication in the United States, United Kingdom and others.
Using UK elections as an example, tactical voting is mainly between The Labour Party and the Liberal Democrats. There may be one district in which the Labour Party and the Conservative Party candidate are running in a tight race, with the Liberal Democrat far behind. In another district, the Liberal Democrat and Conservative candidates may be in a tight race, with the Labour candidate far behind. A Liberal Democrat voter in the first district would agree to vote for the Labour candidate in exchange for a Labour voter from the second district voting for the Liberal Democrat candidate.
Tactical voting has been used since 2000 as a strategy for the U.S. presidential election, with voters from "safe" states voting for third-party candidates, and voters from states with contested races voting for the second-preference candidate of the voters from the third-party. The various people who did vote swapping sites for the 2000 presidential election have banded together as VotePair.org, in the links below.