What are the differences between conductors semiconductors and insulators?

Materials are classified as being conductors, semiconductors, or insulators according to their resistivity.

Resistivity is defined as being 'the resistance of a unit length of a substance of uniform cross section'. In the SI system this equates to the resistance across opposite faces of a one-metre cube of that substance, and is expressed in ohm metres. Note that a one-'metre cube' is not the same thing as a 'cubic metre'.

So, for

conductors: resistivity range 10-8 - 10-6 ohm metres

semiconductors: resistivity range 10-6 - 107 ohm metres

insulators: resistivity range 107 - 1023 ohm metres

To appreciate the difference between these astronomical figures, consider the following:

A 25-mm length of mica (an insulator) of cross-sectional area 2.5 mm2 cross-sectional area will have a resistance of 100 x 1015 ohms. A copper wire, having the same cross-sectional area and resistance, will have a length of 14.29 x 1018 metres! That works out to: 14 290 000 000 000 000 kilometres long!!!!