Scattered among the islands of the Grand Line are mysterious stone blocks called "Poneglyphs."
These blocks are inscribed with the script of an ancient language, and each one tells apiece of a history long forgotten. This history includes the mention of (at least) two weapons of mass destruction: Pluton and Poseidon. There are two types of Poneglyphs: the ones which carries information to reach the other Poneglyphs, which tell the True History and the ones which carries the records of True History. Only when the stones are read as one, they fill in the blank history of the world.
The stones are left in the guardianship of the ancients descendants. Only when the stone has been read by someone willing to take the read passage to the ends of the Earth, are those guarding them allowed to be free of their ancestors wish for them to be protected.
It has been revealed that it is against the law of the One Piece World Government to decipher them. Regardless, to reach and decipher the Rio Poneglyph is the ultimate goal of Nico Robin. Nico Robin states that in order to progress forward, one must take with them the passages they have read from the Poneglyphs they have encountered in their travels to Raftel. It has also been revealed that Gol D. Roger was able to read and write in the language of the poneglyphs. Although Roger's subordinate, Silvers Rayleigh, mentions that he does know what the Void Century is and the mysteries of the Poneglyphs, he admits that saying what it is would take away the impact or any further interpretations of the lost history. He tells Robin to search out the Poneglyphs and draw her own conclusions regarding the history.
The poneglyph tells the location of Pluton. Nico Robin never wanted to give Crocodile the ancient weapon so she lied by telling the history of the land which is not found in the Poneglyph.  Robin was looking for the True History, not Pluton, so she lost her will to continue on. However, she was saved by Luffy.
Filler Alabasta Poneglyph
In the anime filler, Chopper, Luffy and Zoro get lost in the desert somewhere and fall in the hole where the poneglyph is half-buried. However, this poneglyph is not a canon one, as it is only in the anime.
Poneglyph of Shandora
The Poneglyph of Shandora is located in Skypiea. Robin discovered that each location of the Poneglyphs held not one but two Poneglyphs. The first Poneglyph has a message to finding the second Poneglyph, while the second Poneglyph held the message of the lost century. Skypiea's fake poneglyph was located in a set of ruins Robin found in Upper Yard. Skypiea's true Poneglyph was located on its Shandorian Golden Belfry Bell and it revealed to Robin another dark secret -this poneglyph spoke of how to locate yet another weapon, Poseidon. Gol D. Roger had also previously read this Poneglyph and left a message encouraging anyone who could read the Poneglyphs to continue searching for the lost history. The Shandians were left to guard the Poneglyph but over time as events in Skypiea played out they forgot about its existence.
Located in Ohara, this poneglyph includes information about the Lost Century--among other things, it includes the name of the lost civilization, though it has yet to be said what the name is. The World Government considered the archaeologists studying it dangerous and used the Buster Call to destroy the island, Nico Robin being the sole survivor.
The Rio Poneglyph is the non-existant poneglyph, which contains the true history of the One Piece world (including the "Blank Century" that is forbidden to be researched). Once found it will reveal the lost history from the Blank Century. However in order to "find" it, one must take all other passages from the other Poneglyphs with them on their travels.
According to Robin the Rio Poneglyph is the message carried by all the Poneglyphs along the Grand Line and it is only completed upon reaching "the ends of the world", in other words, the island of Raftel which Gold Roger seemed to have done.