The UK uses a first-past-the-post electoral system. For national elections, the UK is split into 650 geographical units, or parliamentary constituencies, of approximately 73'000 people each (these have changed for the upcoming election). Where you live determines which constituency you can vote in.
For each constituency, there will be a list of candidates, normally linked to parties, you can vote for (how to vote). You get to place one vote with one candidate, and the candidate with the most votes (plurality) wins, even if they didn't receive the majority of votes. For example, if a candidate gets 40% of votes, and two other candidates get 30% each, then the candidate with 40% of the votes wins. This then decides who will be the MP for your constituency ('seat').
These 650 MPs form the House of Commons in parliament. If one party has the majority of seats, then they will form a government. If no party gets a majority, then they either need to form a coalition with other parties, or a minority government may be formed.
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