How the same votes produce two completely different parliaments.
Plurality voting: the country is split into districts, and whoever wins the most votes there takes the seat; all other votes are lost. This tends to produce two big parties (US, UK). Proportional representation: seats are allocated by vote share, so 30% of votes yields roughly 30% of seats (Netherlands, Sweden). Germany combines both principles.
Useful: To understand an election result, first understand the electoral system.
Source: en.wikipedia.org