Elections to the Welsh Assembly use the Additional Member System (AMS). This is a way of electing candidates that is able to make sure that the number of seats a party wins is similar to the share of the vote it received.
There are two different type of Assembly Members elected under this system. The first set of seats are won in a traditional first-past-the-post election in local areas, where the candidate with more votes than anybody else wins. In Wales, there are 40 such contests. The second set of seats (20) are won at a larger regional area using a system called ‘proportional representation’, and is designed to ensure that no party wins either far more or far fewer seats than the share of votes they received. There are 5 regions in Wales.
As a voter, you get two ballot papers. The first features candidates in local constituencies, and you have one vote to give to whoever you wish. When counted, the candidate with the most votes wins. You’ll probably be very familiar with voting like this from UK General Elections.
The second ballot paper only features a list of parties, not candidates. This is the regional ‘list’ election. You again have one vote, to give to whichever party you wish, it can be the same as your first vote, it can be different. The votes are counted and election specialists work out what share of the votes each party has got. Let’s say Party A wins 60% of the votes. This is then compared to how many seats Party A has won in the first election. If it does not have enough seats to reflect their share of the vote, the party gets to fill another seat with a candidate from the ‘list’ they submit before the election. For example, Party A received 60% of the votes, but only won 20% of the seats in the first election. Party A therefore is allocated another seat, to take their share of the seats closer to the 60% of the vote that they got. This moves on to the party which got the second most votes, but fewer seats, and is repeated until all of the seats have been filled, sometimes cycling through the parties several times.
This system is designed to avoid a situation where a party receives a large share of the votes in an area but doesn’t actually get many seats (e.g. coming a close second in every constituency but not winning any). The final spread of successful candidates across the parties may not exactly correspond to the share of the votes they got, but it is likely to be close, and in particular, much closer than the ‘First Past the Post’ method used in UK General Elections.