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 - or in short, electrons in motion. Because the valence electrons in metals are relatively free to move about, when you apply a negative charge to 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.

Below is an interactive animation which also demonstrates electrons moving through metal. Click on the image below to begin the activity. Increase the voltage of the battery and observe the differences in electron movement between metal and plastic.

Source: Conductivity,PhET Interactive Simulations, University of Colorado at Boulder