ONE PIECE ROLE-PLAY

Pacifistas are cyborgs that the World Government* created. They're very strong and can shoot lasers (thanks to stealing/experimenting with Kizaru's* powers) and use the body of a very powerful ex-Pirate and Shichibukai*, Bartholemew Kuma.

Void Century was a 100 year period that the World Government has declared taboo to research. During this era there were created the Ponyglyphs* and the Ancient Weapons*. The Government is so paranoid about the era that they even went and annihilated an island, Ohara*, using the dreaded Buster Call*, to the last citizen. In fact, Ohara itself and talk of the place is now considered taboo and the island has been all but forgotten by the world, bar a few people and Nico Robin*.

The World Government is the ruling power of One Piece and consists of an alliance of many, many different islands and powers in the world. It is ruled by the Celestial Dragons* and the Gorosei Five*. They are shown to be full of corrupt people so far, who turn a blind eye to slavery and other horrible acts when it comes to those in power.

Kizaru is one of the three Marine Admirals* and very, very strong. He ate the pika-pika no mi (light-light devil fruit) which gives him the ability to travel at the speed of light and use lasers. He's also seen wielding a sword of light and is one of the strongest characters in the One Piece world.

Shichibukai, also known as the Seven Warlords of the Sea, are seven strong/infamous pirates who work for the Government to aid them in policing the seas. They are the answer to the Yonkou. Over the course of the manga/anime there have been changes to the line-up. The original members consisted of Sir Crocodile, Boa Hancock, Dracule Mihawk, Bartholomew Kuma, Gekko Moria and Donquixote Doflamingo.
Sir Crocodile was eventually replaced by Marshall D. Teach, Gekko Moria went missing after the War of the Best, Bartholomew Kuma was turned into a Pacifista (possibly still counts as a Shichibukai, though), Buggy the Clown took over Marshall D. Teach's position when he quit and Donquixote Doflamingo quit.

Ponyglyphs are indestructible tablets that contain information about the Ancient Weapons and the Void Century. The World Government has tried to destroy or move them, but failed. They are written in a text that has been lost to time and only the scholars of Ohara ever managed to translate it. Spread through-out the world, there is only one person left capable of reading the text and it is her dream to read each one and learn about the Void Century.

The Ancient Weapons are three incredibly powerful weapons that were created during the Void Century and hidden. Their names are known - Poseidon, Pluton and Uranus - and two of the three have been shown in the anime/manga already. Pluton is a machine of some kind. The blueprints for it have been destroyed after being protected and handed down from shipwright master to pupil since the Void Century. Poseidon is an actual person who has the ability to call apon the power of Sea Kings and control them as they will.

Ohara was an island of scholars who investigated any and all interesting things. This includes the Void Century. The island was destroyed by the World Government when it was discovered they were researching the Void Century. Only one person survived the destruction - a young Nico Robin.

Buster Call is the name given to an armada attack by the Marines to destroy an island completely, including all who live on the island. Only the Admirals are supposed to be allowed to call for a Buster Call. Two have been seen in the series so far - the first on Ohara in a flashback, the second on Enies Lobby - the island of justice where pirates are judged before being executed or sent to Impel Down.

Nico Robin in the sole survivor of the Ohara Buster Call and considered one of the most dangerous people to the World Government due to her ability to understand the Ponyglyphs and determination to uncover the Void Century. She has since joined the Mugiwara pirate Crew and has been in contact with the Revolutionary Army. A main character of the series.

Celestial Dragons are a group of people who are thought to be the descendants of those who created the World Government. As such, they are considered Holy and Untouchable - by either the law or others. They tend to be complete asshats and are shown breaking laws, going so far as to buy slaves and kill people in public, take what they want and being spoilt. If any goes against them the Marines send an Admiral to deal with them.
Indeed, their first appearance in the series shows one of them killing a slave that can no longer work, steal a woman with the intent to marry her despite her clearly being shown not wanting this and shooting said woman's fiancé.
They are also clearly inbred-looking. And require a tissue.

Gorosei Five are five elderly men who are the ruling Elders of the World Government. They are the true power of the world. Not much is known about them besides their name and faces.

The Marine Admirals are three of the strongest Marines. They're the second-highest command rank in the Marines, topped only by Fleet Admiral, and are given many powers over the civilians and other Marines alike. They are given codenames that combine a colour and animal. There have been five Admirals shown in the present of the series thus far.
Aokiji - the Blue Pheasant, also known as Kuzan - who has eaten the ice-ice fruit. As a Logia-type his is very strong, being shown freezing a tidal wave and using an ice-sword. He was a contender for Fleet Admiral, but lost the battle that he and fellow Admiral, Akainu, were involved in. He has since left the Marines.
Kizaru - the Yellow Monkey, also known as Borsalino - who uses the pika-pika no mi and still retains his post as an Admiral.
Akainu - the Red Dog, also known as Sakazuki - who used the magma-magma Logia-fruit and has since been upgraded to Fleet Admiral. He has been shown to be very strong, does not use a sword and is very hard-line when it comes to justice. He is also hated by many of the One Piece fan base due to his actions in the War of the Best.
Fujitora - the Pink/Purple Tiger, also known as Issho - who uses a gravity-type fruit. Not much is known about him except that he is blind and seems to want to protect the civilians from the dangers of battles that the marines may cause.
Unknown - there's one more unknown Admiral.

Yonkou are four 'Kings' of the New World. They are the strongest four pirates in the world and considered to be extremely strong with many allies and other crews beneath them. They 'own' various areas of the New World and rule them.
They consist of Kaido, 'Red Haired' Shanks, Big Mam and Whitebeard. After the War of the Best, Whitebeard's allies had to pull out, hence his spot was taken by Blackbeard.

War of the Best was a war to save the life of a captured pirate, one of Whitebeard's sons. Many pirates took part in the effort, as the pirate was well-known and well-loved. Many still, lost their lives and eventually they had to pull out after Red-Hair Shanks arrived on the scene to put an end to the fighting. Betrayal, death, family and the definition of justice were the themes of the arc. Widely considered one of the better arcs (which isn't saying much since imo all the arcs - bar fucking Davy Back - are all decent.)

Sea Kings are giant beasts which look like serpent-animal hybrids most of the time. They roam all areas of the oceans - the Blues, Grand Line and New World. The Calm Belt, which buffers both sides of the Grand Line, is a breeding ground for the beasts and considered the most dangerous area in the world.

Impel Down is the under-sea prison where pirates who have been caught are kept. There are several different 'levels' in the prison, with the bottom most floor reserved for the most dangerous criminals. There have only been two break-outs in the history of the prison - one being a character from the Strong World movie, Shiki. The second was set during the series and involved a break-in of one and break-out of many.

Mugiwara; Strawhat in English; is the moniker given to Monkey D. Luffy, the main character of the series. His crew is called the Mugiwara crew, which consists of - currently - nine characters.

Revolutionary Army is a group of people under the banner of Dragon, the most Wanted person in the world, who work against the World Government and Marines to free countries from tyrannical rule and save slaves. They haven't been talked about much, but it is known that Bartholomew Kuma was, at one point, a Revolutionary.

Fleet Admiral is the highest rank of the Marines. There have been three shown thus far in the series - the ex-Fleet Admiral, Kong; Sengoku the Buddha and Sakazuki.

Devil Fruit is the name given to certain fruits that, when eaten, give powers to the eater. The eater will then be unable to swim or move in water (though their powers will still be active as long as they are already an auto-power) and have a weakness to the mineral, keirosake/Sea Stone. There are three types of Devil Fruit - Logia, Paramecia and Zoan.
Logia allows the user to become an element and control their body as such. They are considered the strongest of the Fruit powers and there seems to be different levels of the same element, or near to. For example, the fire-fire fruit is weaker than the magma-magma fruit despite both dealing with fire-based powers.
Zoan allows the user to become an animal depending on what model fruit they ate. They usually have three modes - fully human, hybrid and fully animal. Some have animal additions such as smell and hearing added to their human form. There are also rarer versions of this category - mystical, ancient and awakened. Mystical/mythical zoans deal with mythical creatures such as phoenixes. Ancient deals with creatures like dinosaurs. Awakened is a very special kind of zoan who has uncovered a hidden power within themselves and can reach a higher form.
Paramecia allows the user to use 'superhuman' powers. This includes control over a certain thing (like wax), changes to the body (rubber or mineral, for example) or special abilities. It's the most common type of fruit and is considered the weakest, despite the fact that all Devil Fruit powers depend on the users' intelligence and ingenuity in use to factor strength.

New World is the latter part of the Grand Line and considered the most dangerous and unknown ocean in the world. It is also considered the area where One Piece is hidden and ruled by the four Yonkou.

One Piece is the McGuffin of the series - a treasure that was hidden by the Pirate King (the first person to sail all the way through the Grand Line from start to end.) and thought to consist of an enormous treasure unlike any other. It was confirmed to exist by Whitebeard during the War of the Best and started the Era of Pirates.

Haki is a power that uses the will of the user to allow certain skills. Broadly, there are two types of Haki common to everyone, given the proper training; however, there is a third type that only a certain group of "chosen ones" are said to possess. In simple terms, Haki is an ability to sense spiritual energy and overpower enemies. See more here: http://onepiece.wikia.com/wiki/Haki