Conductor:
The solids which have high conductivity or low resistivity are called conductors. Metals and most of the alloys fall in this group.
Semiconductor:
There are certain solids called semiconductors whose conductivity lies in between that of conductor and insulator.
Example: Ge, Si, GaAs, CdS, etc.
Insulator:
The substances of this class (quartz, mica, etc) have very high resistivity or very poor conductivity.