Take a 27t stock motor, look inside. See how the windings are attached to a steel armature made up of stacked steel plates called a lamination stack.
Coreless motors don't have that. They use a basket weave of wire that deosn't need support the way standard winds do. So they are lighter and have less rotational inertia giving them faster response.
At least thats what it looked like to me on the motors I've seen. (not servos though)
Just stick to Hitecs 7955, 7950, 7940 and you don't have to think about these things anymore.
You could call a brushless servo motor cordless.
It has no cords or bushes contacting its rotor........
Ready dis to exorcise ye brain:
http://www.electricmotors.machinedes.../bdeee3_2.aspx