There are 2 different types of electric lines for the transportation of electricity.
Transmission lines are for large distances, their voltage is higher, and they can transport more electricity. You can see them on the side of the freeway for example.
Distribution lines are for short distances, their voltage is lower, and they transport electricity locally. You can see them on the side of the street. More information below.
Transmission & distribution simply explained
Power travels from power sources to residential, commercial and industrial customers. During this, the electricity goes through transmission lines and distribution lines. From one type of line to another, the electricity goes through transformers which modify the voltage in order to optimize the energy lost through electromagnetic fields.
Transmission lines are used for interstate connections, for example.
Transmission lines go from power plants to substations at a high voltage. They can bring electricity from one end of the country to the other, using interstate connections. There can also be transmission lines between countries.
In the US, the transmission network looks like this:
The voltage of transmission lines can go anywhere from 69 kV up to 765 kV. The power poles are higher and cables are thicker than ditribution lines.
Transmission lines are bi-directional: the electricity can go in both directions. This is a necessary feature for the balance of the grid. In wholesale electricity markets, electricity flows from areas of generation to areas of demand. But these areas of demand may change over time, so the electricity will travel in different directions.
Transmission networks require operating more actively than distribution networks. Maintenance must be done through automatic and manual interventions.
Distribution lines are used for local electricity delivery.
Distribution lines are low voltage lines which bring electricity from substations to your home. The lines are smaller, and cover shorter distances. The whole distribution network in the US is not directly connected: they are connected through the substations which connect them to the transmission network.
The voltage of distribution lines varies between 4KV and 69KV.
The cylindrical box is a secondary transformer, which brings the voltage down to 120V or 240V.
Distribution lines in the US are traditionally made only to travel in one direction. Therefore the distribution network was not initially designed to accommodate generation. But since distributed generation has started to be implemented on grids worldwide, smaller generation facilities (wind turbines, solar panels, and other localized power generators) are plugged on the distribution network. This causes more stress on the distribution grid, and it is required to renew this grid in some cases, to make it more efficient for these new methods of generation.
As compared to transmission lines, distribution lines are passive systems, (they are not actively managed by operators or computer programs). Also, since they are traditionally unidirectional in power flow (from high voltage to low voltage), they do not require much maintenance. This has started to change in areas where distributed generation is implemented.