The one major mechanical-slash-flavorful thing we know about Khans of Tarkir is that during the time period that the first set of the block takes place in (for this is a block that doesn't take place in a linear time fashion, as it were), the wartorn plane of Tarkir is divided up between five warring clans, each representing a "wedge" tricolor combination. Usually, this means a color and its two enemies, but for Tarkir this represents a color, one allied color, and one enemy color, for reasons that have yet to be explained (though this is the same as the usual idea in most practice).
The clans have a love-hate relationship with Dragons - having killed the species all off, they continue to revere one aspect of the great monsters, and embody that with a symbol drawn from a Dragon body part.
The first we learned about was the Mardu Horde, which is ruled over by the Orc, Zurgo. Once upon a time, this was the home clan of Sarkhan, now a planeswalker, who was Zurgo's trusted general. During Sarkhan's ascension, the entire unit under his authority was slain, and Zurgo thus holds an old grudge against the ex-emissary of Bolas.
The watermark of the Mardu is Dragon wings, and its favored aspect of the Dragon is speed. This is why they are featured as one of the parts of Duel Decks: Speed vs. Cunning. Impatient and quick as they are, red is their "center" color, with white and black supporting. The Mardu appear to be most like the ancient Mongols - hence the name Horde - because they favor a restless lifestyle, always conquering, and conducting fast and brutal attacks from horseback, especially with Berserkers and Archers.
Will you align yourself with the Mardu in the battle for Tarkir? Let me know in the comments.