Ok fellas --

I have been doing quite a bit of reading lately and I think I am getting it but there's one hurdle I can't seem to get my mind over. I know they have ICs with PWM and step/direction inputs etc. Ok, if I have an IC like what I have described, with all that capability, how does that IC interphase with a computer and control software?

What I am asking, to keep it simple, is that if it tells me what pins to set high/low in order to give a step and direction command to the stepper motor, how does the control software utilize my drawing to set the particular pins high/low in order to get the precise cut that I need? Would any software work on these ICs? Does the software have to be customized to a particular IC?

Bear in mind that I have not built or moved anything yet. I have just been doing a great deal of reading trying to understand this thing as thoroughly as possible. Please break things down to the most basic form possible. I need desperately to understand this part of things very very well. Thx