I won't plagiarize here this short 1-page excerpt from the Eberron Campaign Setting. If you can't afford to buy the sourcebook (which I highly recommend), at least visit a library or bookstore and read this short introduction to Eberron on pages 8 and 9 of the ECS.
Dragonmarks are birthmarks in certain shapes, reminiscent of tattoos. Whoever possesses a dragonmark is granted minor spellcasting in the form of a spell-like ability, usually useable once per day. Sometimes a person chooses to learn to use this gift to greater effect, learning more powerful spell-like abilities and increasing the daily uses of lesser abilities.
Dragonmarks only appear in members of the seven races listed in the Player's Handbook. Each mark appears only among a certain race. Long ago, the families who displayed the dragonmarks organized into mercantile houses, guaranteeing that the marks could be used for the financial and political gain of the families to whom they belonged.
New races in Eberron are true-breeding (except warforged, who do not reproduce). This means that they can create other members of their race by having a child with a member of their own race. For example, shifters do not need to be (and indeed almost never are) the offspring of a lycanthrope and a human. Instead, most shifters are born of a shifter mother and a shifter father. This also holds true for existing half-breeds, including half-elves and half-orcs.
Eberron has a strong focus on the craft of magic items. The artificer lives and breathes the power that can be contained in objects. He learns to make all kinds of magic items, and knows how to make any object work to his benefit.