We are playing in the Eberron setting so this is what you need to know.
1. If it exists in the D&D world, then it has a place in Eberron. Eberron is all about using the core elements of the D&D world in new ways and interesting combinations, with some unique elements thrown in. It’s still a D&D setting, so any information for players that appears in another D&D core rulebook or supplement—from the classes and races in a Player’s Handbook to the new powers and other features in a book such as Divine Power—should fit right in to your Dungeon Master’s Eberron campaign. (Of course, your DM always has the final word about what parts of the D&D game are allowed and not allowed in the campaign.)
2. Tone and attitude. Eberron takes all the cinematic action and swashbuckling adventure of traditional D&D games and adds in a strong dose of mystery and scheming. In this campaign, stories don’t always end well, and there isn’t always a right answer to every problem. The Last War turned old allies into bitter enemies and destroyed an entire nation, leaving terrible scars behind. Crime and corruption lurk in the largest cities. Your character’s allies might become his or her enemies in the blink of an eye, and well-known agents of evil might provide assistance when it’s least expected. Hidden dragons shape the course of history. Sinister fiends influence the dreams of the unwary. An army of horrors lingers just beyond the edge of reality, struggling to break through. Nothing is exactly what it seems.
3. A world of magic. The setting supposes a world that developed not through the advancement of science, but by the mastery of magic. Magic allows for conveniences and services undreamed of in traditional medieval fantasy. Bound elemental creatures power elemental airships, rail transport, and high-speed ocean vessels. A working class of minor mages uses ritual magic to provide energy and other necessities in towns and cities. Advances in magic item creation have led to everything from self-propelled farming implements to sentient, free-willed constructs.
4. A world of adventure. From the steaming jungles of Aerenal to the colossal ruins of Xen’drik, from the towering keeps of Sharn to the blasted hills and valleys of the Demon Wastes, Eberron is a world of action and adventure. Adventures can draw your character and your allies from one exotic location to another across nations, continents, and the entire world. The quest for the Mirror of the Seventh Moon might take you from a hidden desert shrine to a ruined castle in the Shadow Marches and finally to a dungeon below the Library of Korranberg. Through the use of magical transportation, your heroes can reach a wider range of environments during an adventure, and thus deal with a diverse assortment of monsters and challenges.
5. The Last War has ended—sort of. The Last War, which plunged the continent of Khorvaire into civil war more than a century ago, ended with the signing of the Treaty of Thronehold and the establishment of twelve recognized nations occupying what was once the kingdom of Galifar. At least overtly, the peace has held for just over a year as the campaign begins. The conflicts, the anger, and the bitter pain of the long war remain, however, and the new nations seek every advantage as they prepare for the next war that they believe will inevitably eventually break out on the continent.
6. The Draconic Prophecy. The dragons, long-lived and patient in all things, seek meaning in the patterns found in the world and the heavens. These patterns play out in the Prophecy, a record of things to come that has been emerging since the creation of the world. The Draconic Prophecy is as complex and unfathomable as the dragons themselves. It hints at events of doom and dread as often as it helps push the world toward exalted events. It seems to point toward transformation rather than destruction, but to most people, the Prophecy remains as alien as the dragons themselves.
7. The Five Nations. The human-dominated civilizations on the continent of Khorvaire trace a lineage to the ancient kingdom of Galifar, which was made up of five distinct regions, or nations. These were Aundair, Breland, Cyre, Karrnath, and Thrane. Four of these nations survive to the present day as independent countries; Cyre was destroyed before the start of the campaign. The devastated territory it once occupied is now known as the Mournland. A common oath or exclamation among the people of Khorvaire is “By the Five Nations,” or some version thereof. The Five Nations refers to the ancient kingdom of Galifar and evokes a legendary time of peace and prosperity.
8. A world of intrigue. The war is over, and the nations of Khorvaire now try to build a new age of peace and prosperity. Ancient threats linger, however, and the world desperately needs heroes to take up the cause. Nations compete on many levels—economic might, political influence, territory, magical power—each looking to maintain or improve its current status by any means short of all-out war. Espionage and sabotage services create big business in certain circles. The dragonmarked houses, temples both pure and corrupt, crime lords, monster gangs, psionic spies, arcane universities, royal orders of knights and wizards, secret societies, sinister masterminds, dragons, and a multitude of organizations and factions jockey for position in the afterglow of the Last War. Eberron teems with conflict and intrigue.
9. Dragonmark dynasties. The great dragonmarked families are the barons of industry and commerce throughout Khorvaire and beyond. Their influence transcends political boundaries, and they remained mostly neutral during the Last War. The heads of each house, not technically citizens of any nation, live in splendor within their enclaves and emporiums located throughout Khorvaire. These dynastic houses of commerce derive their power from the dragonmarks—unique, hereditary arcane sigils—that manifest on certain individuals within the family, granting them limited but very useful magical abilities associated with the trade guilds the family controls. Dragonmarks are said to be the Prophecy written on mortal flesh—a supposition that incenses the dragons.
10. Dragonshards. Ancient legends and creation myths describe Eberron as a world in three parts: the ring above, the subterranean realm below, and the land between. Each of these world sections is tied to a great dragon of legend—Siberys, Khyber, and Eberron, respectively. Each section of the world produces dragonshards, stones and crystals imbued with arcane power. With the aid of dragonshards, dragonmarks become more powerful, elementals are controlled and harnessed, and magic items of all sorts are crafted and shaped. These shards, however, are rare and difficult to come by, making them expensive and often the goals of great quests and adventures.