The immensity of the Mongol empire meant that communication was essential to rule.  To address this problem, the Mongols created a system known as the Yam.  This system of communication is one of the things that greatly impressed the European observers because of it's organization and efficiency.  One of the most valuable sources of information on how it worked was recorded by Marco Polo.  There was many purposes that the yam system served as.  One of the the things that it was designed to do was to help envoys traveling between courts.  The yam also served to help the transportation of goods, especially between north China and Mongolia.  It ensured quick travel of royal orders from one part of the empire to another.  It also served to facilitate the travel of intelligence as quickly as possible.
    The only place that one might begin when searching for information about the yam is as usual, the Secret History of the Mongols.  According to this history, the system was created by Genghis' successor, the Great Khan Ogedei, in 1234.  Parts of the Secret History tell us that the passage before 1234 was a burden because of the risk.  An organized network was thought up, and was supposed to address this problem.
    The responsibility for maintaining the yam system was given to the military units in the various parts of the empire. Everything that was needed for the construction of this network was contributed by the local population.
    The structure of the system was based on a post being built at lengths equivalent to a day's travel.  This means that each post was put up at about 25 or 30 miles.  At these posts, courtiers could obtain fresh horses, food and rest if they had the proper authorization.  The authorization would be a tablet of authority, called a paiza.  The paiza would be made of wood, silver or gold, and sometimes have a tiger in the front.  This all depended on the rank and importance of the holder.  It might take a traveler a day to move 25 miles, but urgent messages could move at a much faster pace.  Marco Polo claimed around 200-300 miles per day.  Express couriers would wear a a bell or or carried a horn that they would sound as they approached a yam station.  A horse was then prepared for the courier, and they could continue on without stopping after changing horses.
    Under Mongol rule and protection, the Silk Road gained travelers.  These travelers and convoys could travel throughout the Mongol Empire without the fear of being attacked.  But in earlier times, travel like this could not have been possible because of the many marauders that lay waiting in the steppes.  The peace that the Mongols kept over the Silk Road helped to make trade flourish.  Thus making it possible for trade to be established between China and Europe for the first time in history.
    The yam system was probably the most effective of the Mongol imperial institutions that came after the army.  Military commanders and governors had to report everything that they learned that was unusual to Genghis Khan as fast as possible.  It was because of the yam that it was possible for the overlords to be kept aware of everything that was happening in their empire.  Without the yam, the Mongol Empire would not have been able to keep control of such a large area of land.

Pictures:

A picture of the paiza that was used to show authority

An actual picture of a paiza