Plurality voting is our current system. Each voter votes for one candidate, and the candidate with the plurality (most votes) wins, regardless of whether that candidate gets a majority or not. In a plurality election with N candidates, a candidate can theoretically win with just over 1/N of the votes. The larger the number of candidates, the smaller the percentage of the votes needed to win. Plurality voting is perfectly adequate when only two candidates are running, but it cannot effectively and fairly accomodate more than two viable candidates. This fact is known as "Duverger's Law", and it explains why the US political system is a two-party duopoly without effective competition from other parties. [Note: plurality voting is also known at "First Past The Post," but we don't consider that label meaningful or useful.]
As most voters know, plurality voting in general elections essentially forces voters to vote for one of the two major parties. Everyone is free to vote for a minor party, of course, but voters who do so usually "waste" their vote on parties and candidates with little or no chance of winning. Supporters of minor parties, therefore, are caught in a dilemma: they can vote sincerely and waste their vote, or they can vote defensively for the "lesser of two evils" as a hedge against the "greater evil."
Take Libertarians and Greens, for example. Libertarians tend to prefer Republicans over Democrats, but Greens tend to prefer the opposite. If either adopts the attitude of "damn the torpedos, full speed ahead" and votes sincerely, they risk hurting their own cause. In the 2000 US Presidential election, for example, voters who voted for Green candidate Ralph Nader failed to help Democrat Al Gore defeat Republican George W. Bush. Hard-core Greens might argue that their votes "sent a message," but many Greens would no doubt gladly retract their "message" to put Gore in the White House.
It is possible, of course, for a popular third-party candidate to gain widespread support. Reform Party candidate Ross Perot, for example, managed to win a respectable 19% of the vote in the 1992 US Presidential election. However, Perot may have tilted the election from Republican George Bush to Democrat Bill Clinton, who beat Bush by 43% to 38% (in the popular vote). In other words, had Perot not run, Bush might have won re-election. If so, plurality voting failed to serve the interests of democracy. Many other examples could also be given of failures of plurality voting.
As ineffective as plurality voting is in gereral elections, morover, it is even worse in primary elections, which is why even Democrats and Republicans should be very interested in dumping it. Primary races often attract several credible candidates. What then typically happens is that the race shapes up as the most prominent candidate against the rest of the pack. The "rest of the pack" then split the vote amongst themselves, and the most prominent candidate wins. The last three Republican Presidential nominees (Bush, Dole, and Bush), for example, were the "highest ranking" or most prominent candidate very early in the race.
See for yourself! Vote and compare the results: