Basic PWM - Step-Dir Questions

Can somebody explain some basic things for me ? I've searched on the
internet and got a lot of conflicting info. What I am interested in is
a basic explanation of PWM (pulse width modulation) and step &
direction signals.

Let's assume I have a basic type CNC system with signals coming from
software on my PC to a controller/driver card and then on to the motor
windings. So the pinouts from the parallel port will send pulses thru
various pins on the port ... a (+ or -) 5V pulse on one pin eventually
runs thru the controller board causing one of the stepper motors to
move one single step (assume no microstepping here). A (+ or -) 5V
pulse on another pin eventually runs thru the controller/driver board
to change the direction of on one of the stepper motors.

So let's say that in the software I have on my PC, I want to turn a
single stepper motor 1 entire revolution clockwise, then reverse
direction and turn 1 entire revolution counter clockwise. Assuming 200
steps per revolution, does the PC first send a single pulse indicating
direction on the dir pin, then 200 pulses on the step pin, then a
pulse on the dir pin to reverse direction, then 200 more pulses to
step the motor around 1 revolution ?

If so, any idea what is the amount of time for a pulse in a standard
system ? Do the pulses have to be a particular time width and
duration, or is there something that controls this ?

Now another question ... based on my reading, to move, say a standard
bipolar motor requires energizing and de-energizing the correct
windings in the motor in the correct sequence. For the sake of
argument, lets say it takes 4 of these different mini-steps to
energize/de-energize the motor to move one single stepper motor full
step. Are these mini-pulses sent by the PC thru the parallel port ? So
now the pulse train looks like 4 mini-pulses for dir, 800 mini-pulses
for stepping, then repeat ? Or are these mini-pulses handled by the
controller/driver board ??

And finally ... I understand the purpose of PWM (to control the amount
of current in the motor windings by rapidly measuring and adjusting
the average current in the motor windings) but how does this relate to
step/dir signals ? How does the low voltage step-dir signal from the
PC get translated into a high voltage signal to the motor ? Is this
done thru the big power transistors like MOSFETs etc. ? If so, how
does the PWM fit into all this ?

Thanks in advance for any light you can shed on this.

Matt