I'll try and minimize the spoilers as well. The story is much better than I can describe with my poor English and without ruining a lot of the suspense.
Mistborn is a dark world where the human race has been saved from disaster by a mighty "wizard" of the sort, who named himself the Lord Ruler. People have forgot exactly what the danger, several centuries ago, has been, but the legend claims the Lord Ruler defeated the Deepness and saved the world. After that, the Ruler united all men under his banner and forged a centuries-long empire, under his sole rule. Rapidly, the Lord Ruler became sort of a tyrant. He promoted people who helped him in his ascension and granted them with parts of the same "magic" that he has been gifted with. He made them mistborn. Others became servants, slaves, known as the skaa...
Several centuries later, the apparently immortal Lord Ruler has a firm grip of the world, which is decaying and dying. Almost all life is gone save for mankind and several tough animal species or vegetation. The skies are permanently cloaked with a dark, thick cloud of smoke from several volcanoes that erupt regularly. The sun can still be seen, but casts a reddish color on the dead land. The landscape has become gray, brown, and rotten. Everything is sooted with ash. Only the nobles can achieve some level of cleanliness and prosperity, through the skaa they make work everyday a little harder. These have become expandable merchandise anyway, with no more value than dogs.
But in the heart of the empire, a man who is supposed to be dead has come back from the grave, apparently, with unnatural powers. Powers that allow him to command metals, to affect emotions, and to enhance his body and senses. Normally, only nobles would be mistborn, wielding this magic that fuels on metals that must be priorly ingested. And even among them, not many are still gifted with the power of mistborn. But a skaa mistborn ... that would be an abomination. This man, Kelsier, soon finds out that there is another one: a street urchin, the illegitimate girl of a nobleman, named Vin. Together, with the help of Kelsier's band, they hope to start a skaa rebellion, and overthrow the power of the Lord Ruler.