It won't win awards for the graphics, but the gameplay is so complex it is almost a simulator.
Building units doesn't require you to spend resources like many other games. Building units uses up your Production, determined by the Industrial Capacity sliders, and also takes away from your Manpower, which is the amount of men who you can recruit for war determined by your population. HoI2's units aren't single soldiers, or squads of soldiers. The game deals with ground units in Divisions, air units with Squadrons, and sea units with single ships. Units' states are not shown as their health, but their strength out of 100% and their morale. Units also require supplies, and if they are low on supplies, then they will be less effective. Supplying units requires that the territory they are in is in the supply line. If they are isolated from the supply line, then attacking with those units would be scuicide, since units can't fight without their ammunition.
Battles are not as complicated as other aspects of the game. Battle outcomes are decided by various attributes, such as morale and the amount of supplies, but as mentioned before, units are controlled in large groups. So, instead of telling A squad to go here, you tell the units in X territory to attack Y territory. Thats it. Units in territories ajecent to the defending territory can support the attack, which is helping your attacking units in battle but when you capture the territory, they don't have to move into the captured territory like most units. Air units can do all sorts of things, depending on their type- fighters can patrol over territories, and you can assign bombers to attack enemy indistrial centers. Naval units can bombard and attack enemy convoys.
And then there's diplomacy and government. There's all sorts of actions you can do with that. Diplomacy and government in HoI2 in this game is the most incredibly complicated part of any game I've ever played in my life. When you initiate di-plomacy, there are 22 different actions you can do, and they are available during certain situations; for example, you cannot propose an alliance when you are already allied. There are 6 sliders that change the policies of your government, which can be influenced by events or other nations. These sliders also determine your type of government. Nations of similar governments will be closer than those with different governments.
Basically, with HoI2, if you want to do something, you probably can.
Hearts of Iron is an amazingly complicated and deep game by itself, but the game also has a lot of single-player modes to play in. First off, you can play as any country- Not just those that participated in WWII, I mean any country in the world, from Nicaragua to Ethiopia to Australia to Cuba to Germany. That is, with the exception with puppet states, but thats a good thing, because puppet states' militaries are controlled by their controllers. I can't say most of them are worth playing since the complicated political system makes it unbelieveably difficult just to declare war, but also that most wouldn't have the industrial capabilities to be able to wage the war. There are a ton of scenarios you can play in as well; the focus is the open-ended scenario where you chose a year, nation, and then you are thrown into a virtual Earth during the WWII era. Descisions are made by you, and there are various events dotting the game. There are also scenarios where the game focuses on a specific battle, such as Operation Barbarossa where Germany invades Russia and the Ardennes Offensive where Germany cut off the 101st Airborne. There's plenty of single-player modes for years of play.
In HoI2, there is nothing different between the single and multi-player games, except that you have a human playing as one of the countries. You still have all the scenarios, all the countries, and all the technologies. However, having another human playing changes the whole game drastically. As an ally, invasions can be easily coordinated and planned, while a cunning human enemy can turn international relations against you. Multiplayer has the same features a single player, except with the added effect of another human being playing. Unfortunately, there aren't many games playing, and those who are are usually passworded. One really awesome thing about HoI2 is that you can play the game- single or multiplayer- without the CD. You can install this game on any of your friends' computers, and considering its low system requirements, it'll probably work. You could set up some great games going with those you know, like some of you do with games like Diplomacy or Risk.
HoI2's Graphics... well, let's face it. There isn't any. This game was made board game style, and it shows. There are no three dimesntions. Ever. Everything is two-dimensional. Although tank units are shown as tanks and air units are shown as planes, every nation his its own graphic for each type of unit. So although German and US tanks will be different, you won't tell the difference from different US tanks just by looking at them. For a board game, they are fine. For a video game, they just suck.