Like Carzjr said; a slotted motor has windings wrapped around iron (usually laminated) cores. A non-slotted motor is basically a "basket" of wire inserted into a cylinder core and the wires don't wrap around a core.
I would think a slotted design would be better since it concentrates the magnetic fields and thus increases torque. But
this page seems to think otherwise. What they say seems to make sense about the cogg-free characteristics since both Neus and IIRC Novak use slotted motors and those have issues on the MM. They also say that slotless motors can be more efficient ( heat up less), are smoother running, etc. I wonder if Neu motors would be better if they were slotless while maintaining the higher end quality? I guess then they would be Lehner. : )