ITs computer to motion devices ether by USB, Ethernet or parallel port connection to the computer.
the motion devices can be the bob (breakout board) and motion device (look up Gecko G540 it's a all in one) and or a stepper driver or just a motion devices that connects to a bob.
It's all down to your budget and how much you want the motion devices to do.
A G540 is a good option to start with it's got good backup and support, It's all you will needed.
A ESS and a bob and stepper drivers is fine like what daz or joe has
I have the same as joe on one machine it has G214v gecko drivers. it works fine
on another I have A G540 what I had working with the parallel port on mach3 and 4 it was fine, then I got a Pokeys 57cnc and connected the G540 to it, it's a good machine so I spend the money to keep it like that.
Have a read of the info here
http://www.geckodrive.com/support/step-motor-basics.html, there is some good info there and here as well
http://warp9td.com/index.php/gettingstarted