Conductors- Those substance which conduct or they pass electricity thorugh themselves, they are called Conductors.
Ex- Silver, gold  or copper.

Insulators- Those substance which do not allow to pass the electricity at any cost
they are called Insulators.

Ex. Wood, plastic .

Semiconductos -Those substance which allow some of the electricity to pass through them, they are called Semi-Conductors.

Ex- Silicon etc.
Conductors are those substances in which there is an overlap between valence band and conduction band (i.e almost no energy difference between valence and conduction band) so that electrons can easily move from valence band to conduction band and conduct electricity. For ex- Metals
In case of Semiconductors, there is small energy difference between valence band and conduction band(≈1eV, in Ge it is 0.78eV & in Si, it is 1.21eV) so that on applying electric field, that amount of energy is easily acquired by electrons in order to move from valence band to conduction band. Eg- n-type & p-type semiconductors
Insulators are those substances in which there is large energy difference with respect to electrons present in valence band and conduction band & which cannot be achieved by electrons pressent in valence band from any source. 
The energy difference is called forbidden energy gap(For eg- Diamond, 6.6eV)