“The grid” refers to the electrical power grid that connects all consumers of electricity to all of its suppliers. It is a complex connection network.

The grid is made of two primary types of pieces:

• Transmission lines (in multiple different voltages), with their substations, transformers, and interconnections – they constitute a network.
• Power plants that provide electricity to this network.

Source: Wikipedia

All electrical equipment connected to the grid is synchronous, in the sense that it operates on exactly the same timing with the respect of the alternative current (AC) it receives.

The topology of the grid (i.e. the exact form of the network and location/power of the plants connected to it) dictates how the grid may be managed. In the US, the grid is really a set of three regional grids that have limited interconnections. What this means is that a region can not really import or export much power to/from another region. Within each region, there may be many grid operators that manage different parts of the grid. The US grid is tightly coupled with the Canadian grid, and loosely coupled with the Mexican grid.

At all times, demand on the grid (which is mostly not controlled by the grid operators) must be matched by supply. Grid operators make that happen in real time, all over the country, by continuously managing how much power is provided into the grid, through turning different plants on and off, or tuning their output.

The assets on the grid are owned and/or by different parties. Independent parties may own transmission line and/or generation plants, and generate income through agreed-upon system rules. Many of these rules are state- or region-specific.

The distribution of power to the individual consumer is typically a monopoly (a local utility company), whose operation is tightly regulated by its state authorities. Some utility companies only own power plants, with no direct distribution outside of the global grid. There are thousands of electric utilities in the US.

This interactive map, from NPR, provides five different and fascinating views of the grid.