If glass rod is an insulator, how can it be used for charging by contact? Charging by contact requires conductors. When a positively charged glass rod touches a neutral metal body, the body gets positive charge. But how is it possible as glass rod is insulator? 
 A: The charge and voltage on an object like the glass rod are related by:
$$ C = \frac{Q}{V} $$
where $C$ is the capacitance. For a glass rod the capacitance is very small so the charge $Q$ will be very small and the voltage $V$ will be very high. This means the current required to redistribute the charge is very small and there is a high voltage available to drive that current, so even if the resistance of the glass is very high the charge will still flow on the timescale of a few seconds. That's why the charge can flow off the glass rod and onto the metal body.
To work out how fast the charge will flow requires you to specify the geometry of the rod and the charge and voltage on it. The resistance of the glass will depend on how clean it is and the relative humidity. A glass surface adsorbs water from the atmosphere to form a (slightly) conducting surface layer.
A: Does electric charge move from or inside an insulator?
It's possible, but it's slow.   No 'insulator' has zero conductivity, just
very low.   And, no object in our everyday lives has a perfectly clean
surface; many kinds of dirt, oxides, or moisture films are conductive.
Even air is conductive enough to dissipate charge in time (but if we sort
socks quickly, they crackle).
High voltage helps, too; a van de Graaff generator of the fills-a-building
type might use a several-kilovolt power source to spray charge from metal
electrodes onto a rubber belt.  The rubber doesn't have to be a perfect insulator, just has to be motor-driven at a faster (feet/second) speed than electric
charge can move along its length.
