There are actually several DIFFERENT games, which have words "il-2 sturmovik" in their title.
The oldest and the cheapest is original IL-2 Sturmovik Forgotten Battles+addons (from Pacific Fighter to 1946) by 1C Maddox Games. This forum is for that game. This game is by now 11 years old basically, but still is updated with free add-ons.
There is also a sequel to that game from 1C, called IL-2 Sturmovik Cliffs of Dover. It's about Battle of Britain, built on new modern graphics engine, etc.
And a side spin-off by Gaijin Games, called IL-2 Sturmovik Wings of Prey. This one is a kind of "relaxed realism" flight sim, also with better graphics, then the original one.