Short answers, pulled from the story.
A multi-party system is a political system where more than two meaningfully distinct political parties regularly run for office and win seats in elections. No single party typically achieves a parliamentary majority on its own, so multiple parties must negotiate to form a coalition government.
Duverger's law is a principle in political science stating that proportional representation electoral systems tend to produce multi-party legislatures, while winner-take-all systems tend to consolidate around fewer parties. It explains why countries with proportional representation are more likely to have multi-party systems.
When no single party wins a parliamentary majority, multiple parties negotiate to form a coalition, sometimes called a minority government, that can command a majority of votes in the legislative chamber. This coalition majority is required to pass laws, form an executive government, and carry out the basic work of the parliament.
A two-party system is a special case where only two parties have a realistic chance of winning, requiring voters to align into two large blocks. Multi-party systems allow more than two parties to win office, encouraging smaller and more ideologically defined groups to compete independently rather than merging before election day.
Countries with multi-party systems include Argentina, Belgium, Brazil, France, Germany, India, Israel, Italy, the Netherlands, Norway, Sweden, and Ukraine, among others. These countries span multiple continents and political traditions but share the structural feature that more than two parties regularly win office.
Multi-party systems tend to discourage polarization by requiring coalition-building before governing is possible. Because no single party can make major changes without support from other elected parties, they are strongly motivated to negotiate and compromise, which moderates governmental decision-making and discourages ideological drift toward the extremes.