How it all works
The Scottish Assembly consists of 139 members, of which 73 are chosen by the traditional first-past-the-post method, in constituencies corresponding to the constituencies of the 73 Scottish MPs who sit in the House of Commons.
The remaining 56 members are chosen by a form of proportional representation. They are selected from party lists, which exist in each of the country's eight electoral regions, which correspond with the European parliament boundaries in Scotland. The system is called the Additional Member System.
There are thus two ballot papers: one for a candidate in the local constituency, and one for a party in the region. The system allows people to vote for both a party and a person, the lack of which duality is a criticism often made of both first-past-the-post and closed-list elections.
First-past-the-post simply means the candidate who polls the most votes in the constituency wins the seat.
However, the Additional Member system is more involved. Voters vote for a 'party list', which is simply a list of each party's candidates in that region, with the most important candidates near the top (to give them the most chance of being elected.) This can be anything in number from a single person upwards. If a candidate is standing in both a constituency and is on the regional party list, the constituency takes precedence: if he/she wins their constituency, their name is removed from the party list.
The d'Hondt system, in use throughout PR countries in Europe and in Northern Ireland, is used to find which candidates should win seats from the top-up or party list. It works as follows....
This is repeated until all seven additional members are elected. The aim is to compensate parties which get a large proportion of the vote, but fail to win any seats.
I hope that has explained it. If you need any further clarification, e-mail me and I'll be happy to help.