What is voltage?

Voltage You May Think It Is The Energy,It Is The Soul Of The Circuit,Voltage Produce The Difference potential (電位),Than The High Potential Will Flow Through The Loading(LED,Bulb...),And Than Back To The Low Potential.
To Drive The Electron Is The Voltage,It CAN NOT Control The Speed,
To Control The Speed Of Electron Is The Control The Amperage,It Can UseResistor To Control The Speed(Amperage),
potential difference Is 電位差,It Mean The Difference of Two Point,Such As(1.5V,0V),(220V,0V).
Hope This Help.

No. The speed of electrons in a conductor is NOT light speed. In fact, it is much much lower than light speed, even lower than the speed of sound.
From simple theory, current I in a conductor is given by
I = n.e.v.A
where n is the number of free electrons in the conductor per unit volume (electron number density).
e is the electronic charge.
v is the speed of electron (commonly called the mean drift velocity)
A is the cross-sectional area of the conductor.
Since by Ohm's Law, I = V/R
where V is the voltage applied to the conductor and R is the resistance of the conductor
hence, V/R = n.e.v.A
that is to say, for a given conductor, the speed of electron v is proportional to the applied voltage V.
Potentilal difference is simply the difference in voltages between any two points in a circuit.

actually, the speed of electron in conductor as light speed. Higher voltage is not higher speed!

Definition: Voltage is a representation of the electric potential energy per unit charge.

If a unit of electrical charge were placed in a location, the voltage indicates the potential energy of it at that point. In other words, it is a measurement of the energy contained within an electric field, or an electric circuit, at a given point.