Ok, wall of text incoming.
It's a flight simulator. You can go to
www.il2sturmovik.com to check out screenshots etc.
At this point it contains 200+ flyable planes plus some AI only ones. You can also play offline, it has a built-in dynamic campaign generator plus you can build your own scripted missions or full campaigns, so there's always something new to try out. Online you can either play on a dogfight arena (up to 64 players, respawn upon getting killed) or in cooperative missions (up to 32 players plus AI planes, no respawn until mission ends).
It's not for the faint of heart though. I'm lucky that when i started playing flight sims on my first PC the technology prevented full realism, so i got introduced to the difficult elements gradually in a period of years and i still suck compared to most online players. Difficulty is scalable when you play offline, but online it's determined by server settings and most good ones are full realistic on physics. From then on it's mostly if they allow exterior cameras or not. I tend to play in servers which disable external views. It's more difficult for me as well, but at least i can sneak up on people from their blind spots without them pressing the external camera button and seeing me whenever i go.
There's a shitload of expansions and patches out, but if you want to try it out head to a game store and buy the IL-2: 1946 DVD. It contains everything released up to now in a single package and you'll only need one patch to go online.
It contains the following:
IL-2 Forgotten Battles (Eastern front): Germany, Gungary, USSR, Poland, Finland, Chekoslovakia
Aces Expansion Pack (Western front): Limited non-historical maps so no campaigns but you can download user made ones which substitute similar maps for parts of England and France, plus you have most of the british and american planes, plus the first German jet, another one that didn't fly and a rocket powered interceptor.
Pacific fighters (self explanatory)r: Japanese army and navy, US navy, army and marines, British navy and aircraft carriers with maps ranging from New Guinea and Malaysia (this one done with satelite data and 1950-era photos) to Pearl Harbor and Iwojima.
Pe-2 addon: A russian twin-engined plane addon.
Sturmoviks over Manchuria: USSR vs Japan, paid addon.
IL-1946: Fictional "what if" scenario with a bunch of UFO planes and early soviet jets.
These are the paid addons, when a patch came out it usually contained new flyable planes for free. The good thing is that for 30 Euros (45 for the collector's edition with a movie DVD showing some aerobatic online teams and their next upcoming project about battle of Britain) you can get all of it in a single DVD, download the latest 4.08m version patch, a client that finds servers and go online. There will be one more final patch and then they will stop releasing new stuff to focus on the next project, which will be out around autumn.
