"Yakuza" (ヤクザ) is a general title that refers to Japanese crime organizations. As a word it can be used in either plural or singular reference, such as describing one individual, or the organization as a whole.  Similar in scope of operation to the American and Italian Mafia, the Japanese Yakuza is responsible for a variety of both legitimate (such as various agricultural and charitble works) and illegal business activities (prostitution, drug, and arms smuggling to name a few),  albeit with a much higher presence and level of acceptance in public society. Famous for elaborate, full body tattooes ("irezumi" or 刺青 in Japanese), the various Yakuza groups hold equally complicated codes of honor, conduct, and rituals.
Historically, many Yakuza clans trace their family lineages far back into the feudal periods of 1700s Japan, with some affiliations claiming direct connections to a warrior class known as Ronin.  Other names and titles for the organized crime elements of Japan also exist, such as Boryokudan (an unfavourable handle, popular with law enforcement agencies), and ninkyo dentai (popular with admirers, supporters, and actual members of the Yakuza itself).
- Yakuza (Wikipedia)
- How the Yakuza Works (howstuffworks.com)
- The Yakuza, the Japanese Mafia (The Crime Library)