Short answers, pulled from the story.
Capcom vs. SNK: Millennium Fight 2000 is a 2000 crossover fighting game developed and published by Capcom for arcades running on Sega's NAOMI hardware, later ported to the Dreamcast. It is the second game in the SNK vs. Capcom series and the first in that series released for arcades.
Capcom vs. SNK: Millennium Fight 2000 was released in 2000 for arcades on Sega's NAOMI hardware. A revised arcade version, Capcom vs. SNK Pro, reached Dreamcast in 2001 and PlayStation in 2002.
Capcom vs. SNK: Millennium Fight 2000 uses a ratio system rating each character's strength from 1 to 4, with team ratios required to total exactly 4. Players also choose a groove, either the SNK Groove based on the Extra mode from The King of Fighters '94 through '98 or the Capcom Groove based on the Street Fighter Alpha series.
Leon S. Kennedy and Mega Man were both intended to be playable characters in Capcom vs. SNK: Millennium Fight 2000 but were cut so the roster could stay focused on fighting-game characters.
Metacritic gave the Dreamcast version of Capcom vs. SNK generally favorable reviews, while the PlayStation version of the Pro edition received mixed or average reviews. Famitsu scored the Dreamcast version of both Capcom vs. SNK and the Pro edition at 30 out of 40 each, and the PlayStation version at 27 out of 40.
The Dreamcast version of Capcom vs. SNK won Best Fighting Game at GameSpot's Best and Worst of 2000 Awards. The arcade version was also named the most-successful arcade game of the month in Game Machine's issue dated the 1st of October 2000, though it lost Monthly Arcadia's Best Head-to-Head Arcade Fighting Game award to Guilty Gear X.