it is difficult to describe. it is a real time strategy, but not like the other RTS's out there. it is kind of like civilization that takes place only during world war 2. you view the screen as a provincial map. you choose between the different views of the map you want to see. there's a terrain map with military units, a diplomatic map that color coats the provinces by what countries you control, etc. you have to research a huge tech tree and manage 5 or so different resources. you diplomatically manipulate other countries. it is incredibly complex. i could spend hours trying to explain it. here is the website:
http://www.paradoxplaza.com/heartsofiron.asp