If you cut into an electric extension cord, you would notice that the cord is actually metal wires (usually copper) wrapped in vinyl layers. This is because metals are excellent conductors of electricity.

Source: Cut extension cord, Between naps on the porch

Electricity is energy created by the free or controlled movement of charged particles such as electrons. In other words, electricity is energy created by electrons in motion. Because the valence electrons in metals are relatively free to move about, when you apply a negative charge to the end of a piece of metal and a positive charge to the other end, the free (delocalized) electrons move away from the negative charge and toward the positive charge. You can see this feature in the animations below.

Click on the animation to see how electricity is created.

Below is an interactive animation that also demonstrates electrons moving through metal. Click on the "click to run" to begin the interactive. Increase the voltage of the battery and observe the differences in electron movement between metal and plastic.

Click to Run