Conductors

The resistance of a metallic conductor decreases with temperature. The easy way to remember this is to think of electronics which stop working when they overheat.

The resistance of a semiconductor increases with temperature.

Superconductors have 0 resistance below a critical temperature.

It is possible to classify every type of material into these categories. Insulators, when possible to measure, are found to increase in conductivity when heated - so they can also be called semiconductors.

A-level teaches the equation:

Conductivity is the reciprocal of resistivity. A siemen (S) is defined as the reciprocal of an ohm. Using that, you should be able to tell why conductivity can use the units S cm^-1.