Lock On: Modern Air Combat was the original
Lock On: Flaming Cliffs 1 was the Su-25T addon to that.
Digital Combat Simulator:- Ka-50 Black Shark was a standalone sim using a vastly upgraded Lock on engine.
Lock On: Flaming Cliffs 2 :- was the Lock On planes brought into this upgraded engine. (If you have LOMAC, FC1 is not needed for FC2 BTW)
DCS:- A-10C Warthog :- Is a further evolution of what went before.
As for just being one plane, the DCS line of simulations is a whole order of magnitude deeper than Lock On. Every system is modelled for that plane, every single system, unlike Lock On which was fairly simplistic. The Startup vid in the above posts should give you an idea. Of course, if you don't want to have this complexity there are options to turn it off.
The A-10C is based off a simulator they did for the US Air Force, so it's pretty much as real as it gets.
The best bit about DCS is that if you own DCS: Black Shark, you get all the upgrades & improvements of DCS: Warthog (except the A-10C itself) for free in a patch. Lock on however is not going to be evolved further, it's finished after FC2 AFAIK.