What are the main electoral systems?

The winning candidate must secure an absolute majority of the vote.
Candidates are usually elected in single-member constituencies.
The output of FPTP could be described as majoritarian because the voters who’s candidates didn’t win are not reflected in Parliament.
In 2015, 50% of all votes go to a losing candidate and 24% also went to a candidate that had already secured enough votes to win.

Produces a close fit between votes and seats, although no system can deliver perfect proportionality.
The district magnitude is crucial — the larger the constituency, the more proportional the result.
PR systems use multi-member constituencies and electoral formulas to match the percentage of votes won by each party to the percentage of votes they won.
Some (e.g. the single transferable vote) allow electors to vote for as many candidates as they wish in order of preference, whereas others (e.g. the closed list system) permit only a single vote.
Elections to Scottish local government (councils) use an electoral system called the Single Transferable Vote (STV). STV was first used in Scotland in 2007.

FPTP is a single-member plurality system in which the winner needs only a plurality of votes cast, not an absolute majority.
Plurality systems share characteristics of majoritarian systems in that (in the case of the UK) the plurality system produces a majoritarian outcome.
In 2010, when 2/3 of MPs did not gain a majority in their constituency and it consequently allows parties to form a government with only a small share of the vote.
Produces a disproportional outcome.

Combines elements of the plurality or majoritarian systems with elements of proportional representation.
g. AMS. Some representatives are elected in single-member constituencies using first-past-the-post.
The remainder are elected by proportional representation in multi-member constituencies.
Seats are allocated to parties on corrective lines to represent their share of the vote proportionally
Used in Scottish parliament and Welsh Assembly.