|The animation above is slightly deceptive because it shows the electrons as being stationary when there is no voltage.
In reality, even when no current flows through a piece of copper, the free electrons are moving rapidly about. Their speed is about 106 m s-1; that's 3000 times the speed of sound in air! However, since they are moving at random, there is no net flow of electrons in any particular direction and so there is no current.
When a voltage is applied, the electrons gain an additional velocity, so that there is a net flow along the wire. This extra velocity is called their drift velocity. Here's a way to picture this:
Think of a swarm of bees. They are all milling around the hive. Each bee is moving, but the swarm stays still. Now one sets off and the rest follow. The swarm is still a milling mass, but overall it moves away from the hive. The free electrons in a metal are like the bees; it takes a voltage to make the mass of electrons move through the wire.