Magnets on the shaft essentially.
A 2 pole motor typically has a cylindrical magnet with one north & one south pole on opposite sides.
A 4 pole motor tends to have 4 separate magnets glued to the shaft, going N-S-N-S around the shaft. Same deal really with 6 & 8 pole rotors, just more magnets.
http://www.stefanv.com/rcstuff/qf200212.html How motors work
2pole
4 pole ( kinda hard to see the magnets as the rotor is wrapped in kevlar, but you can just make out the face of each magnet, and you can also see the slots inside the can )