The Poneglyphs are giant cube Structures with a ancient text inscribed on them. They tell of two things, the true Poneglyph(all the Poneglyphs put together) tells the history of the Void Century, 100 years of lost history, and the three ancient weapons,Pluton,Poseidon,and Uranus, made in the Void Century that have the power to destroy the world, this is why Robin is so sought after, she's the only one that can read the ancient text, and the only one who can awaken the weapons.
The Devil Fruits are mysterious fruits that when eaten, will give extraordinary powers, but will also take away your ability to swim and float. lots of people in the One Piece world have heard of or eaten a Devil Fruit.
Gol D. Roger
Gol D. Roger, who most of the time was called Gold D. Roger is the father of Luffy's brother, Ace(no pic for him, sorry). during his life, Roger collected vast wealth,power, and knowledge and was named The King of Pirates. When he was finally caught and executed, he told the world he hid all his riches (which is called The One Piece,hence the title) and that it was up for grabs.This started the Great Pirate Era , and every pirate in the world started to look for One Piece, to become the next King of the Pirates.
Straw Hat Pirates
All the current members of the Straw Hat Pirates(excluding Luffy,the captain.)
Rorona Zoro was a world famous bounty hunter until he met Luffy and joined the Straw Hat Pirates as its frist crew member, Zoro is a swordsman that uses three katanas which is known as "three sword style", He set out to sea to find a man named Mihawk, the greatest swordsman in the world, so that he can fight him, and take his title.
Usopp grew up in a small village in the East Blue,His mother died of illness and his father Yasopp, set off to sea as a sniper on Red Haired Shanks pirate ship, and even though his father left him and his mother, he was still prod to have pirates blood, the blood of a brave warrior of the sea, flowing through his viens.In his village Usopp is known as the village liar, running into the town everyday crying, "pirates are coming!". After helping to stop a plot to destroy the village by the pirate Captain Kuro, he joins the Straw Hat Crew as the ship Sniper, on their new ship the Going Merry.
Nami story is a hard one, she grew up in Cocoyasi Village in the East blue with her adopted mother,Bell-mere(a Navy/Marine veteran) and her adopted sister Nojiko on Bell-mere's tangerine orchard.Then the infamous Arlong Pirates, A fearsome group of fishmen, invaded and took over the island, killing Bell-mere right in front of Nami and her sister, Nami, around the age of 10 makes a deal with Arlong,the captain,to let her join his crew, make maps for him(Nami is one of the best navigators/chart maker in the one piece world) and that she could buy back her village for 100,000,000 Berries(currency in One Piece). After Luffy defeats Arlong, she joins Luffy's crew as their navigator.(She travels with the Straw Hats before this but after her village is saved she officially joins the crew)
Tony Tony Chopper
Chopper is actually a reindeer from the Drum Kingdom in the Grand Line, when he was little Chopper ate the Hito Hito no Mi or the Human-Human Fruit, a Zoan (animal transformation) Devil fruit for animals which gives them the power of speech and lets them take on a human-like appearance and other transformations, each having its own strengths and weaknesses.Unfortunately Chopper looked more liked a monster then he did a human, and was rejected by the people there, except for two doctors,Dr. Hiluluk, the quack doctor of Drum Kingdom, he taught Chopper everything he knew as a doctor until he committed suicide to stop the evil king of Drum, Wapol, who was trying to kill all the doctors (except his own) in the Kingdom. Chopper then studied under Dr. Kureha, a friend of Hiluluk until he joined Luffy as the doctor of the Straw Hat Pirates
Robin is an archeologist that ate the Hana Hana no Mi or the Flower-Flower Fruit, which gives her the ability to sprout extra limbs and body parts like eyes and ears like when flowers bloom.She joins the Straw Hats after leaving the country of Alabasta on the Grand Line. She is a well known fugitive who has had a 80,000 belly(Berī) bounty for being a "Demon of Ohara" an archeologist from the island of Ohara in the West Blue, archeologists there could decipher the Poneglyphs.(see Poneglyphs for more info)
Cutty Flam (Franky)
Franky was a street thug on the streets of Water 7, Grand Line.Being the second tallest member of the Straw Hats, Franky is a cyborg and the ships shipwright. His weapons and other robotic features make him a powerful member of the crew. He learned how to be a shipwright by Tom,the best ship wright in the world,the same man who made Gol D. Rogers ships, The Oro Jackson. His dream is to see the ship he made for the Straw Hats, The Thousand Sunny, sail all around the world just like the Oro Jackson.
Brook was not always a skeleton, he was originally the second captain of the Rumbar Pirates, after the frist one died, who have been long dead. The reason Brook is still kicking as a skeleton today is that when he was still alive he ate the Yomi Yomi no Mi or the Revive-Revive Fruit, which brought him back from the dead as a skeleton, then while a skeleton a Shichibukai (Warlord, see Warlords/Shichibukai) by the name of Gekko Moriah stole his shadow (which without it, you burn up in the sun) After Luffy defeats Moriah and reclaims brooks and a lot of other peoples shadows, Brook joins the crew as a Musician
Princess Nefertari Vivi
Although Vivi and her pet duck(which is very large and Really fast for a duck)Carue was only part of the Straw Hats for a short time, vivi is there for Luffy's bingeing adventures.Vivi is the Princess of Alabasta, an island country in the Grand Line. When the Shichibukai Crocodile attempts to take over the country using his criminal organization Baroque Works, Vivi infiltrates and joins Baroque Works to try and stop them on the inside, while undercover she's found out and asks Luffy(which she had met at Whiskey Peak, one of the frist islands you come across in the Grand Line)to take her to Alabasta and help stop crocodile. He agrees and after a couple of in-between adventures(and Chooper's introduction)they arrive at Alabast where Luffy defeats Crocodile and helps Vivi save her kingdom.