紅白

Literally means red & white in Japanese, and can be used to describe something with those colors. However it is usually used to describe a competition between two teams (a red team and a white team). If the teams are divided by gender the men will be the white team, and the women will be red team. Although kouhaku can be used in a number of situations, if you were to mention "kohaku" to a Japanese person they would most likely think of...

紅白歌合戦

The annual Kohaku uta gassen (red & white music competition), usually referred to as kohaku for short. Kohaku is an annual televised music show broadcast on New Year's Eve by the NHK. The Red (women) team and White (men) team, comprised of popular singers of the past year, take turns singing songs. In the end celebrity judges and the audience (both live and at home) vote on the winning team. The show ends with all the contestants singing the Japanese version of Auld lang syne (Hotaru no Hikari), before going off the air at a quarter to midnight.

The show's roots can be traced back to 1945, when a NHK producer thought that the show would provide some light in poverty stricken post war Japan. The occupying forces would not allow the show to be called Kohaku uta gassen (gassen also meaning battle), so the show was called Kohaku uta shiai (red & white music match) and was broadcast by NHK radio. The first Kouhaku uta gassen took place on January 3, 1951, in front of a live studio audience. The show moved to New Year's Eve and television in 1953. The show grew every year and by the mid-1960 the show had become a Japanese mainstay boasting ratings well over 70%. It was during this time that the show settled on it's current format of the first half of the show featuring popular music (for young people) and the second half featuring enka and other traditional music (for older viewers). However as the show went into the 1980's rating began to dip into the 50's. The decline was caused in part by the broader musical tastes of the younger generation, popular rock artists shunned the show as being too bubbly and commercial. The greater options available to families on New Year's eve also contributed to the decline, as many families chose to go on vacation rather than stay at home and watch TV. As the show entered the 1990's the rating for the first half of the show hover around 35-42%, while the second half produces ratings of around 48-51%. These rating continue to this day.

The first Kohaku uta gassen featured 7 singers to a side, while the last Kohaku uta gassen (2003) featured 31 acts per team. The NHK chooses the contestants through a series of surveys to the general public and internal surveys. The NHK exercises complete control of the final selections. Many acts with less than clean images often get passed over, so as not to clash with the NHK's conservative image. This selection process generally produces a line-up of manufactured pop groups (like SMAP) and a core of enka veterans (many of the enka singers have participated in 30 or more kohaku).

While the Kohaku uta gassen has lost its luster over the years, it is still is a tradition in many Japanese households.

Sources:
http://www1.plala.or.jp/nakaatsu/ (Japanese)
The Encyclopedia of Japanese Pop Culture (1997)
Tapes of various kohaku (1995-2003)