A type of semiproportional voting system. Voters are given one vote for each candidate running, and may use them as they see fit. You might put all your votes on one person, or split them between your three favorites. This gives a much better representation of what the people want, as you no longer feel that you have to limit yourself to only voting for people who might win, nor do you have to make those hard choices between two great candidates. On the other hand, this can make for bulky and sometimes confusing ballots. You'll also notice that it could easily fall back into a competition between only two candidates if it were clear that they were likely to be neck-to-neck and well out in front. No one would want to waste votes on minorities when your 17th vote might be the one to push the leader over the top….
At the time of this noding, no country uses the cumulative vote to elect their national legislature. It is used in several cities and counties in Alabama to elect their legislative bodies, and some towns in Texas use it to elect either their local school boards or city councils.
Compare to Limited Vote, another type of semi proportional vote.
You might also look at Alternative Vote and Approval Vote.