When there are three or more candidates, the winner might not reflect the preferences of the majority.
Voters then feel pressured to vote tactically for one of the two candidates they predict are most likely to win, because a vote for any other candidate will likely be wasted and have no impact on the final result.
When we rank restaurants, movies or albums out of five stars, we're implicitly saying that we like some more than others.
Instead of restricting votes to a single favourite, modern ballots ask for more information. They're still looking for the same thing though: voter preferences.
If voters express more than single preferences, we can't simply tally the votes per candidate. There are many alternatives, each with their own relative strengths and weaknesses.
This site makes use of Schulze STV, a form of proportional representation that resists tactical voting. For more information, see Wikipedia's article.