Like most D&D campaigns, Eberron is set among the ruins of a fallen empire, that once spanned the continent and had some magic items and stuff, and is now no more. Unlike most D&D campaigns, the empire fell just a century ago, and people were still fighting over the remains until a good two years ago. The vibe here is post-WWI, with all these soldiers back from the war and not sure what to do, the different countries in an uneasy peace and eyeing each other suspiciously, and all these weird magical weapons having been developed and people talking about how war will never be the same again.
But it’s also got all the crazy pulp-action stuff you expect from the 1930s: south of the main continent of Khorvaire is the jungle continent of Xen’drik, covered in ruins from mysterious ancient civilizations, prowled by sinister dark elves riding scorpions and armed with wicked metal boomerangs. And even weirder stuff. Back on Khorvaire, there’s the Lightning Rail, a magical train whose coaches levitate above the tracks so it can zip along, and fire-elemental-powered airships that soar high in the sky. If you think you may end up fighting on at least one of these while the vehicle is in motion, you are probably correct. There are mad cultists of The Dragon Below who plot sinister schemes of chaos, and at least a half-dozen other groups seeking world domination. And don’t even get me started on the crazy wizards who study the planes as they shift in their transdimensional orbits, waiting for the right conjunction to power their infernal machines.
Another important thing about Eberron is that it’s partly an attempt to extrapolate a world where you have low-level magic available on a large scale. In a not-too-shocking turn of events, it ends up replicating a lot of technology: besides the aforementioned train and zeppelin-analogues, you have magical lamps on the streets of the big city, magewrights working on crafting fine items and breeding magical animals, and then there’s all the stuff that got built for the war. Most notably, the warforged: these guys are like intelligent golems made from living wood and metal, created to be the perfect soldier. For the entire war they were considered to be slaves or property, but part of the armistice agreement gave them full citizenship rights and stuff. This doesn’t mean that the average guy on the street accepts them, though.
Finally, the last thing to know is about dragonmarks. See, in Eberron you have the countries that are, obviously, important entities. But almost as important are the twelve Houses. Each House is mostly run by a very large extended family. Why? Because the power of the House comes from dragonmarks — mysterious symbols that some people are born with, and get passed down erratically through family lines. People with those symbols have the intrinsic ability to use certain magic powers — people with the Mark of Healing can heal wounds and cure diseases, and people with the Mark of Storm can control the weather.
The Houses have parlayed these powers into various guilds, which in turn have enormous influence across the continent. For instance, House Jorasco’s Mark of Healing let them start the Healer’s guild, which runs virtually all the hospitals. House Orien’s Mark of Passage gives them the guilds of Couriers and Transportion, and if you want mercenaries you talk to the folks with the Mark of Sentinel. Obviously, there are healers who don’t belong to House Jorasco (clerics, for instance), but Eberron doesn’t have a lot of high-level characters (like, a quarter or fewer of what a standard setting has), so the powers of the House dragonmarks makes them like IBM or US Steel (and there’s been no trust-busting attempts, at least not yet). Note that not everyone in a House has a dragonmark: there’s plenty of room in the House for administrators, diplomats, bookkeepers, janitors, and so on, depending on your other talents.