The World Government is a really deep and influential organization in One Piece that drives the plot of the series. Created 800 years ago by the alliance of 20 kingdoms, it oversees more than 170 affiliated nations today, which quite literally makes it the ruling entity over most of the world. A federation like this would hold a colossally great political and military power and sway over the One Piece world.
The World Government is complex, having a number of key organizations. The Marines work mainly as its military and law enforcement division. Cipher Pol takes the role as the secret service to the hidden operations. The Judiciary system also works under the World Government to put their laws into action. This complex system makes it possible for the World Government to be in control over so many things in society.
Actually, the World Government does a lot of things; for example, it has hidden too many secrets since its creation centuries ago. It would most probably be related to some mysteries known as the "Void Century": one century in the history of the One Piece world which the World Government has constantly tried to keep away from the eyes of the public. This secrecy adds mystery and intrigue to the way this organization is depicted in the series.
At the top of the hierarchy is the Gorosei, literally meaning the Five Elders. These five individuals represent the peak of the authority of the World Government. If one views from the standpoint of individuals' strength, the World Government is blessed with quite a few powerful people in their arsenal. Some notable members are Figarland Garling, Shepherd Ju Peter, Topman Warcury, and Marcus Mars.
One Piece had an important antagonist known as the World Government. It serves as a persistent obstacle to the protagonist of the series, Monkey D. Luffy, and other characters that oppose its rule. But again, it cannot be said that the World Government is Luffy's arch-nemesis. As one fan pointed out, Luffy doesn't aim to defeat the whole World Government but to get through the troubles it presents in his goal of becoming a pirate.