Power lines are wires that conduct electricity through to another place. Many have transformers. The original power is made at a power plant. The voltage of the power at the end is usually 100V (Japan), 120V (North and Central America, parts of South America and Africa, and Saudi Arabia), or 220-240V (most of the rest of the world). "Step-up" transformers increase the voltage in electric power distribution grids, and increase it greatly in long electric power transmission lines. The power at the end is "stepped-down" by another transformer so it can be distributed and used.
