Is the speaker impedance constant or variable?

We read lots of tech specs on the speakers and know that sometimes speaker impedance may vary from 2 to 6 ohms or some other values.
Does that mean that this impedance vary within one speaker or different speakers of the same brand/model might have differences?
Does the speaker impedance vary on different freequencies or it has pure active and constant resistance? Another words what are the components of the speaker impedance? Equivalent circuit diagram link or e-mail will be helpful.