It models many types of aircraft and weapons for dozens of countries, from early Korean War jets to the latest models of the 1990's. Differences in performance, weapons, sensors, and signatures are accurately modeled, and you must take these differences into account if you are to win engagements, battles, and campaigns.
Each game-turn represents a fixed amount of time, and for each of the aircraft on your side, you must assign an action (typically a turn, maneuver, or weapon launch), using an on-screen joystick, throttle, and buttons. After your turn, aircraft are moved, sensors are checked, weapons are made available to fire (if within parameters), and the opponent replies, moving its aircraft in response. It takes longer to explain than to do -- in fact, even without the available keyboard shortcuts, the turn cycles are very streamlined and intuitive. FC2 is easy to learn and play, but like any good game, not always that easy to win!
The nice thing for flight sim fans is that any sim-derived knowledge of air combat will apply directly to FC2.
You may know from USNF 97 (or other sims, or from reading) that the Vietnam-era F-4 Phantom was an energy fighter, that it lacked guns in some models, and that its Sparrow and Sidewinder missiles had serious limitations. This knowledge will help you greatly in FC2 when you are faced with MiG-17 and MiG-21 fighters -- angles fighters who will out-turn and beat the F-4 if it is not flown correctly. You have to use correct tactics, and it's especially interesting to try various multi-aircraft team tactics such as pincers or drags in addition to boom-and-zoom vs. turn-and-burn.
FC2 comes with single air battles from Korea, Vietnam, Desert Storm -- even scenarios from the movie "Top Gun!" There are campaigns consisting of linked missions in which resource and crew management are also important factors. It includes a mission generator that gives endless variety, with user choices as well as random elements. You can change several settings that affect realism and difficulty. There's a nice little reference library, and the manual is quite good as well. AI is surprisingly good -- it knows the strengths and weaknesses of all the aircraft and plays accordingly (though it will make mistakes on easier settings).
MiGMan thanks Chino for this overview.