Nature of LinucCNC

I am a total newbie to Linux so some very basic confusion on my part.

Is LinuxCNC an application program that runs on top of a generic linux operating system of my choice, and if so how does it install a RTOS in that op system?

OR

Is it an operating system with an RTOS that installs itself in a new partition and an application program that then installs and runs on that operating system?
and if so,
What distribution is that operating system based on?

Do all distributions use the same linux kernel and the same GNU (except for the specific release version) or do different distributions modify the kernel and the GNU included in them or even substitute something else in their place?

LInuxcnc is a program that has to be ran on a special real time OS. (for machine control) It can however be ran for simulation purposes on an ordinary version of Linux.

The real time OS is an ordinary version of Linux who's kernel (the main under pinnings of the OS) has been patched (modified) to provide a real time environment for programs requiring it.

Right now Linuxcnc supports two different versions of real time patched kernels, RTAI and Preempt-RT. Depending on your situation, and hardware you want to control, one or the other may be preferred.

Thanks - a very concise and to the point answer.

1) Can this LinuxCNC RTOS with the patched kernel also run any other normal Linux programs as would any other normal Linux OS distro that had a kernel that was not patched to be an RTOS?

Point is: Do I need a second partition with a normal Linux install to operate generic Linux programs (like CAD and CAM) on this same machine?

2) My applications are a knee mill and an engine lathe. The lathe will have closed loop DC servos on all axes with an AC inverter spindle drive and quadrature type spindle position and axis position encoders. The mill will have closed loop DC servos with analog tachs and digital quadrature encoders on the table X and Y and the spindle Z, an AC inverter spindle drive and quadrature encoders for spindle rotational position, a tilting rotary table with open loop step motors on tilt and rotation axes, and possibly a DC servo with position encoder on the knee Z axis. I am concerned about the mix of DC servo and step motor drive types. Is this reasonably doable in LinuxCNC?

3) Is a digital step and direction output signal the only output available from LinuxCNC or is there a method by which the target axis positions and velocities can be output directly as digital data and then input to a D/A converter to generate the analog drive input velocity signal in a more direct manner?

Point is: the path generator must be calculating the desired position and velocity for each axis. If you are going to use DC servos, it seems pretty silly to then generate the PWM step and direction signals only to have to convert them back to a velocity signal for input to the servo amps. You waste processor cycles generating the PWM signal and more time converting it back to an analog signal in add on hardware, you slow response times, and you incur additional error possibilities in the conversion process.

1) To the best of my knowlage you can run any normal Linux programs along side Linuxcnc on a real time patched OS. The real time patch creates a processing layer underneath the normal Linux kernal that the realtime programs. The normal Linux stuff just runs on top of that using what ever processing power is left over after the real time calculations are done.

2) Not a problem

3). Yes there are several manufacturers of hardware for supporting controlling analog servos with Linuxcnc. In fact Linuxcnc was controling analog servos before it did step/dir control.
Here is a link to some hardware options. (list is not up to date.)
wiki.linuxcnc.org/cgi-bin/wiki.pl?LinuxCNC_Supported_Hardware