The Ronin is a prestige class in Complete Warrior, specifically meant for players using the Samurai class, also in the same book. Ronin are master-less samurai, whether by dishonor and disgrace or by some other act of misfortune, the Samurai has a stain on his code of honor that prevents him from having a lord.
Playing a Ronin
The only difference between a Samurai and a Ronin is their employment. Though in game mechanics, a master-less samurai doesn't have to take levels in Ronin to be a Ronin, but a Samurai who has effectively become dishonored loses his Kiai Smite ability. Only by taking levels in Ronin is the Samurai able to gain back some of his former classes abilities.