Attention! Destroy All Enemies! -– Yes, Sir!
Great Generals are one of the two main features in Warlords. As it can be guessed from their name, they're a new type of Great People, providing corresponding military benefits. The mechanism of obtaining Generals is rather similar to that of obtaining regular Great People. You accumulate General points, and when you have enough, you get a General and the point threshold increases. It should be noted that the point pools for Great Generals and other Great People are not linked – getting a General won't affect your Great People points, and so on.
The General points are gained from combat. When one of your units gets some XP, that amount of XP is added to your Great General point count. If you have the Imperialistic trait, then twice as many points are added. Same stands true if you have built the Great Wall, for battles that occur within it. On Normal speed, you need 30 points for your first Great General – which should means 15 regular battles won. The actual number will probably be a bit different.
Generals have three abilities, not dissimilar to other Great People. They are:
Build a military academy – performed in a city, this action grants +25% military unit production for that city. Or, in other words, it becomes a “build 4 units, the 5th is for free” deal. I like, personally, to do this in my main production city early in the game. I find that with the Heroic Epic added later, on Normal speed military units can be cranked out in 1-2 turns from that city. Even on Epic, it's quite plausible to get 2-turn production times for military units.
Settle as military instructor – the General will settle in the city, like other Great People can, providing +2 XP to all units built in said city. It is also a good option, though the gains really are long-term. Over the course of time, however, the benefit of an extra 2 XP for many units becomes tangible. Note that the Military Instructor will allow you to build units with 2 promotions upon completion even if you are not running Vassalage or Theocracy, you only need a Barracks.
A particularly attractive option is to use the Military Instructor early with the Mongols. Mongol mounted units, with a Stable and a Ger, get produced with 7 XP. Add the Military Instructor, and you have fresh 9 XP units. That seems like a very viable option for massing some Keshiks, even at the earlier stages of the game, when Theocracy and Vassalage are not yet available. These units are powerful, and just one battle (or retreat) away from gaining their third promotion. With Genghis Khan's Imperialistic trait, which allows you to gain your first General very quickly, this seems to me to be one of the best strategies. You can apply a very early Woodsman Warrior or Archer choke to a nearby AI, getting some valuable General points, prepare for a Keshik rush, and just crush the opposition.
Finally, Great Generals can lead your units as Warlords. You “attach” the Warlord to one of your units. At that time, units in the tile get 20 XP distributed among them. Only one unit can become a Warlord, though. You can give this Warlord all 20 XP, or sprinkle several units with XP – a good option, for example, in city defence, to give three defenders a new promotion.