The underworld we talk about here is not to be confused with the place where people are believed to go after they die, but the general term for organized crime. It is a shady world, unknown to most people, where illegal things like murder, smuggling, drug dealing and similar are happening daily. Groups known to be acting in this world are for example the Mafia or the yakuza.
Underworld — Subcategories
A bounty hunter captures fugitives for a monetary reward (bounty). For example the crew in Cowboy Bebop are bounty hunters.
The Mafia is a secret crime group working in many parts of the world that was created in Italy more than 200 years ago. The Mafia make money from crime. They make billions of dollars a year from such crimes as making and selling illegal drugs, money laundering, from stealing, gambling and prostitution.
Yakuza, also known as the Japanese Mafia, refers to traditional organized crime groups in Japan and members thereof. They have a hierarchical structure, with the "oyabun", the family head, standing on top. They also have their own code, the "jingi", where loyalty and respect are a way of life.
Yakuza members are identifiable for their, often full-body, tattoos and missing fingers, a result of the yubitsume ritual, which is a form of penance or apology.