common rail means the engine has a fuel rail so the injectors share a low pressure, and each one works as high pressure pump.

non common rail is when the engine has a single high pressure fuel pump that distributes fuel to the injectors at high pressure.

This in my mind can be cosidered a "mechanical" set up with all injectors doing pretty much the same, where as at the common rail systems every injector works indipendetly from the others, and as such, can be cotrolled indipendently.

Thats how trucks work and I don't think cars are much different in filosophy.