To eloborate on the 'infantry' point, there are several kinds of foot troops - 'infantry' is the catch-all term for the basic division. There are also marines, mountaineers, paratroopers, militia, garrisons, motorized infantry, mechanised infantry and cavalry. I can't remember if Air Cavalry are an upgrade along the cavalry line or a separate unit altogether..

As you can see, the units are abstracted but they have to be because of the size of the game. As well, different countries also follow different lines of tactics , which increase their units' abilities in certain areas, like night-fighting, for example.