Knockout Kings gives the user a chance to compete against 32 real fighters, such as Muhammad Ali, Marvin Hagler, Sugar Ray Leonard, Oscar De La Hoya, Larry Holmes, Jake LaMotta, Roberto Durán, Alexis Argüello, and Ray Mancini. In the game's "slugfest" mode, fighters can knock each other out at any given moment of the fight, or suffer 6 or 7 knockdowns and still last for the whole fight.
There are several differences between Knockout Kings and real-life boxing careers. Championship bouts are only for 10 rounds and not for 12 as in real life. In the career mode of Knockout Kings, a fighter is made to retire right after winning the title and defending it for only the first time. Also, Knockout Kings does not have women's boxing. The exception being the PlayStation version of Knockout Kings 2001.
Before releasing Knockout Kings, Electronic Arts released its first 3D boxing game, Foes of Ali, for the 3D0 in 1995. While there are a few similarities between the two games, Foes of Ali was developed by a different team, Gray Matter and as such, is not regarded as a true prequel.
Source: Wikipedia, "Knockout Kings", available under the CC-BY-SA License.