For those of you that aren't acquainted with the NPB Playoff system, it's quite different from its American counterpart. There is one big series at the end called the Japan Series (similar to the World Series of the MLB). And there are two main leagues that make up the NPB system. But that's pretty much where the similarities end. But before I get into the Playoff system, let me quickly touch on the basic definitions of the NPB.
Both the Pacific League and the Central League are made up of 6 teams. The Central League has similar rules to the National League in that there is no DH. Likewise, the Pacific League has the DH, just like the American League. From what I can tell, there is really no set number of games the teams play per season, but it usually seems as though it's anywhere from 130 to a 140+ games (although I do remember reading somewhere that there are talks to set the number of games for CL teams to 146 and 140 games for the PL). And because each league is somewhat autonomous, the teams from the different leagues might end up playing a different number of total games. I'm sure most are already familiar with the rule that allows ties in Japanese baseball -- any game that remains tied after 12 innings will end as a tie.
So that's the two leagues in a quick nutshell. And now for an explanation of the Playoff system.
As I mentioned a little earlier, the two leagues in Japan are somewhat autonomous. This extends into the manner in which the top teams are picked for each league. The Central League currently does not have a Playoff system and sends the top team to the Nippon Series. The Pacific League, on the other hand, has a Playoff system (introduced in 2004) that involves the top 3 teams of the league -- the first round involves pits the 2nd and 3rd place teams in a best of 3 series; and the second round involves the winner of the first round and the 1st place team in a best of 5 series. Incidentally, the last two seasons have actually seen the second place team of the PL become Nippon Series Champs. And the Nippon Series is a best of seven matchup.
There is apparently some talk that there will be some refining of the playoff schedule for the 2007 season, but nothing concrete that I have heard of so far.
And that, my friends, is the how the playoffs are played in Japan... at least for the time being.