As u/dillpiccolol says the Mongol military was organised in a decimal system. Genghis Khan at the beginning of his conquest (after uniting the Mongol nation in approximately 1206) appointed 97 Minghaan officers. These were in charge of 1000 men each. 10 minghaans made up a tumen, these were large forces that could be sent off by Genghis independently due to their competent commander. Each minghaan would have 10 jaghun commanders commanding 100 men, these were appointed by the minghaan leader. Within a jaghun there would be units of 10 (arbans) who would each elect their own leader. An arban would ideally be self sufficient, so had to carry supplies and shelter for 10 men. This decimal system was not revolutionary from Genghis Khan as it had been used before, but not on such a scale. As each commander had to pass orders onto only ten men it meant that there was a fluid command structure during battle. Even down to the lowest arban, commanders were expected to show initiative and had some freedom to carry out objectives.

That was the basic Mongol military structure, obviously each unit was rarely actually totally full, Tumens that would in theory have 10 thousand men would more likely contain around 6,000-8,000. Genghis was renowned for his meritocratic structure whereby birthright was influential, but a simple tribesman with no noble blood (Subedei) ended up being effectively supreme commander. Promotions were based very much on skill and this helped to propel the 'Horde' forward.

My source for this answer is predominantly Timothy May's 'The Mongol Art of War', although any book about Genghis Khan will usually go into military structure with some depth. I'd be happy to answer any further questions you might have.

To further the response from /u/dillpiccolol about the organization or decimal system, I would like to add the following:

Units were built from groups of 10 (Arban), 100 (Yaghun), 1,000 (Minghan), and 10,000 (Tumen), each with a leader reporting to the next higher level. Tumens, and sometimes Minghans, were commanded by a Noyan, who was often given the task to administrate specific conquered territories. From two to five Tumens would then form a horde - that is, an army corps or field army, under the command of the Khans or their generals (boyan).

In note to why the feigned retreat was possible, six of every ten Mongol troopers were light cavalry horse archers, the remaining four were more heavily armored and armed lancers. Mongol light cavalry were extremely light troops compared their european counterparts allowing them to execute tactics and maneuvers that would have been impractical for a heavier enemy (such as European knights).

Another point is that the Mongolian horses are relatively small, so they would lose short distance races under equal conditions. Since most other armies carried much heavier armor, the Mongols could still outrun most enemy horsemen in battle. In addition, the Mongolian horses are extremely endurable and sturdy, which allowed the Mongols to move over large distances quickly, often surprising enemies that had expected them to arrive only days if not weeks later.

Another good example of the Mongols ability to perform was the battle of the Kalka River, which I can't really sum up without making this much longer, but still, check it out.

Edit: My previous point about stirrups was incorrect regarding the Mongols, as pointed out by /u/LordBojangles so please disregard it - by their time it was widely used in Europe.

The Mongols extensivly used stirrups to turn their upper body, and shoot in all directions, including backwards, allowing them to harass the enemy as they rode away, a rather brilliant tactic that could enrage pursurers and lure them even further into a trap.

Sources: Ghenghis Khan and the Mongol Conquests, Stephen Turnbill. (And a number of other sources, that I cannot remember atm sorry, If you wish I could try to dig them out.)

As a reminder, since we've had to remove several replies to this question already, please consult our rules of answers. In particular, answers should be in-depth and more than just book or podcast recommendations.

I'm from Mongolia and we have had this Chingis khan mania here for a while, since the break up of ussr, in fact. Now i understand why it's happening, with soviets trying to suppress nationalism coupled with the indignation of being under manchus, i suppose we don't really have anything to be proud of and here is our chance finally, Chingis khan, the man of the millenium, 16 million descendants and all that bollocks!

What i'masking is , was he really that special? Wasn't he just another bloodthirsty central asian warlord? Yes , he united mongolian warring tribes, so what?!

And why is it that he is constantly in the top 10 of the most brutal people in history? Can someone source me on that please, (books, podcasts, anything) so i can show my fellow mongolians where the numbers are coming from. So far all i get is this excuse, we were illiterate therefore our history was written by others, our cruelty embellished etc.

Not sure what this subs consensus on him is, but I'd really recommend Dan Carlins "wrath of the khans" 4 part podcast if you're interested in learning about the mongols. Guy makes history really interesting to listen to