Type of Game
Grand Strategy on a truly Global Scale. Pick any country in the world and you can probably take control of it in HOI2.
You can start the game from as far back as 1936 - nothing major happens during this time but we all know what is round the corner. Do you use this time to build up for defense / attack or do you follow what actually happened in real life (it will soon all merge into one believe me). Diplomacy is a key part in this game - you can make or break alliances or do you join an existing one such as the Allies / Axis. Joining one of these grants you certain bonuses, the leaders very rarely let "the little one" go without. They provide materials / reseach blue prints (to help speed up your own research) and sometimes even let you command their armies as an expiditionary force - a massive help in your own expansion.

You can control land/ sea and air forces with household names like the U.S.S Enterprise, Sherman tanks and FW-190's - too many to mention =) Weather can take a part in battles - raining and you cannot rely on your Air Power. Attacking a mountainous area you will struggle due to the digging in of the defenders.

Best Parts
Research
Each country has their own unique research teams. For example the UK has Hawker, Supermarine, Avro etc - all household names for the WW2 buff. Each team has its own strengths and weaknesses. When you select what area you wish to research it is also broken down into what is needed, and you can simply match up the best team with that tech. Obviously, picking an Air Research team like those above, wil greatly reduce their efficiency if you want them to research the next level of Submarine for example. Also with research, it gives you a rough guide as to when in the real world a particular item was widepsdread developed. This stops the player rushing towards Atomic bombs in 1936 - youve got to play within the limits

Worst Parts
Depth
It took me approx 6 weeks to finally understand everything in the game. The tutorials are not that good and the guide book you get with it is about the same. So many intricate details to oversee and control a new player can feel out of depth. Perserve - it does get a lot better - then you will not want to put the game down