A CoreXY printer refers to a mechanical arrangement for a Cartesian printer style.
A CoreXY printer is described in full details here at RepRap.org.
A CoreXY uses a so-called parallel manipulator system with stationary steppers. This printer type typically has a lower inertia than other arrangements that carry a stepper or steppers in the moving parts (serial stackup). A lower inertia results in the ability to use higher acceleration and higher speeds. This reference shows some excellent figures on the layout of a CoreXY printer.
