The Underworld of One Piece is filled with this huge network of illegal activities, suspicious dealings, and smuggling that stay beyond the peg of influence by the world government. This clandestine system plays a very integral role in the series as it influences events and power dynamics throughout the entire world of One Piece.
At the top of the Underworld hierarchy are the Underworld Emperors, otherwise referred to as the "Emperors of the Underworld" or "Yonko of the Underworld". The people holding these positions have immense influence and power over this dark world. Among the famous Underworld Emperors, there are "Big News" Morgans, who earned his money through newspapers; "Drug Peclo," a drug lord; and Du Feld, also titled the "God of Fortune" and the "Loan Shark King.".
Brokers are the middlemen for most transactions and exchanges of information in Underworld. Currently, ten big broker parties are known to exist, out of whom four identities have been revealed: Eustass Kid, Pekoms/Tamago representing Big Mom, Rob Lucci, and Sheepshead. These brokers establish links between the different factions of this Underworld and run the bigger schemes.
One of the strongest Underworld characters, Donquixote Doflamingo, popularly known as the Joker, had been so prior to his defeat at the hands of Monkey D. Luffy. It had been behind almost all of the Underworld activities, particularly in the manufacture and supply of artificial devil fruits called SMILEs, prior to his fall at the hands of Monkey D. Luffy. His fall handed an immense power vacuum among those that were left within the Underworld ranks.
The reach of the Underworld stretches as far as arms dealing, human trafficking, and the bartering of rare or banned merchandise in the world of One Piece. This would very often cross into the plot of the major story arcs by the selling of weapons between warring factions or the trading of information of great value.
The interesting thing would be that, in One Piece, "underworld" could very much apply in terms of a dual meaning. While it is primarily associated with a criminal network, some fans are speculating that there may be a literal underworld, probably involving something about souls or even the afterlife. Of course, this speculation partially relies on Brook explaining his soul's condition right after his death. But this spiritual aspect of the Underworld wasn't deeply developed within the main story.
The Underworld is situated within the New World, the latter half of the Grand Line. Most of its activities are based there, in this dangerous ocean where the influence of the World Government comes in more lax. There is also mention of an area called the "Underworld of the Sea" deep into the ocean, relating to these illegal activities, but little is known of this area.
As many of us know, generally speaking, underworld activities in anime and manga are very secretive. Characters usually hold meetings, clandestine exchanges, and shadowy transactions during such scenes. The characters speak in code, use den den mushi for secure communication, and meet in hidden places to avoid authorities.
In the world of One Piece, Underworld operations extend their influence to almost every event, from shaping the power balance between great factions and the information flow to determining the outcome of wars. Many times, one could say that something as simple as equipping weapons or providing intelligence via Underworld channels drastically changes the nature of battles and political struggles.