Electoral system which is supposed to divide up the seats in numbers proportional to the popular support for all the parties. Many systems exist - in the Scottish Parliament we have single seat constituencies elected by FirstPastThePost and top-up lists for groups of geographically close constituencies. Parties nominate lists of candidates for the regional seats and additional members are elected from these lists to make each party's total number of members approximately proportional to their popular vote across the regions. Each voter casts one vote for their constituency MSP and one for the regional list. I like this system: it meant I could vote for a major party as my MSP and cast a sort of ProtestVote for the GreenParty? in the regional list.
In the Netherlands, we have proportional representation. There are 150 seats in the "Tweede Kamer" (comparable to British House of Commons) to be divided. They are divided proportionally to the number of votes for each party, with some complicated round-off rules. This means that 1/150th of the total electoral votes will get your party a seat in Parliament. There are indeed several parties with only a few seats.