A conductor is a material which allows electric current to pass through it.
Metals such as gold, copper, iron etc, are normally good conductors but there are also non-metallic conductors e.g. graphite (the inner part of your pencils).

An insulator is a material which does not allow electric current to pass through it. Insulators are: wood, paper, glass etc.

A semiconductor is a material which under different conditions either allows electric current to pass through it, or prevents electric current from passing.
Semiconductors are: silicon, germanium with infinitesimal impurities of other materials such as indium, gallium, arsenic, phosphorus, etc. which alter their electrical behavior