Cable Anatomy I: Understanding the Microphone Cable
by ProCo
This article will help everyone understand a little more about wire and microphone cable construction, and to explain why the good stuff costs a little more.
Some technical articles for your enjoyment. These were written by the Pro Co engineering division to help everyone understand a little more about wire and cable construction, and to explain why the good stuff costs a little more. You can download all three articles which are in Adobe Acrobat format(.PDF). The PDF files also contain some graphs and pictures and as long as Pro Co is given credit for them, feel free to use them if you wish.
What is impedance? 
Impedance is the AC (alternating current) version of the DC (direct current) term resistance, which is the opposition to electron current flow in a circuit and is expressed in ohms. Impedance (often abbreviated as “Z”) includes capactive reactance and inductive reactance in addition to simple DC resistance. Reactance depends upon the frequency of the signal flowing in the circuit. Capactive reactance increases as frequency decreases: inductive reactance increases as frequency increases. Because of this frequency dependence, impedance is not directly measurable with a multimeter as
DC resistance is.