The Characteristic Impedance depends upon the ratio of the values of the capacitance per metre and inductance per metre. To understand its meaning, consider a very long run of cable that stretches away towards infinity from a signal source. The source transmits signals down the cable which vanish off into the distance. Since the far end is a long way away, the signals transmitted from the source can't initially be influence by the properties of any destination before they finally arrive. The result, when the signal power vanishes, never to be seen again, is that the cable behaves like a resistive load of an effective resistance set by the cable itself.
Engineers use a variety of type and impedances of cables.
At RF, the use of 300 twin feed is fairly common, and 600 is often used at audio frequencies. Standard values tend to be adopted for convenience in a given application area as this makes it easier for people to build systems from compatible elements. University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.