The resistance is related to a DC circuit. The impedance is related to an AC circuit.When we talk about impedance we should consider capacitive and inductive elements in the circuit. Measuring the impedance of a speaker driver will show a difference between the DC value of the voice coil and measurements taken at different frequencies showing the behaviour of the inductive part of the coil in action and the capacitive part of the whole. The DC measurement is taken when, for exemple the speaker driver's cone doesn't move (static) while the AC measurement of impedance is taken with a sweep of frequencies (10 Hz to 10k Hz). A 8 ohms impedance woofer may have 6,5 ohms of DC resistance. I hope it helps to clarify the difference.