Short answers, pulled from the story.
SNK vs. Capcom: The Match of the Millennium was developed and published by SNK for the Neo Geo Pocket Color in 1999. It remains the best-selling game in the entire Neo Geo Pocket and Neo Geo Pocket Color library, with 75,000 copies sold in Japan.
The game was developed by SNK after director Takashi Nishiyama met with Capcom's head of development Yoshiki Okamoto to propose a collaboration. The two then convinced their respective company presidents, SNK's Eikichi Kawasaki and Capcom's Kenzo Tsujimoto, to cooperate. Art director Toyohisa Tanabe noted the series was partly intended to generate software for the struggling Neo Geo Pocket Color.
IGN gave the game a perfect 10 out of 10 on the 5th of January 2000. Famitsu magazine also rated it positively, scoring it 30 out of 40 on release.
The game features 26 characters drawn from SNK and Capcom franchises. Some characters, including Jubei, Arthur, Marco, and Fio, appear only in Olympic Mode minigames, while Billy Kane and Vega make non-playable appearances.
Olympic mode is a collection of themed minigames, including a first-person shooter segment lifted from the final boss of Metal Slug 2 and a platforming game featuring Arthur from Ghosts 'n Goblins. Points earned in these minigames can unlock special attacks and, via link-up cable, extra content in other games including Capcom vs. SNK: Millennium Fight 2000 for the Dreamcast.
A Nintendo Switch port released on the 17th of February 2021, adding local multiplayer and TV mode support for the first time; it was later included in Neo Geo Pocket Color Selection Vol. 1. A Windows port followed on the 30th of September 2021.