An electric grid is a network of synchronized power providers and consumers that are connected by transmission and distribution lines and operated by one or more control centers. When most people talk about the power "grid," they're referring to the transmission system for electricity.
The continental United States does not have a national grid. Instead, there are three grids: the Eastern Interconnect, the Western Interconnect and the Texas Interconnect. In Alaska and Hawaii, several smaller systems interconnect parts of each state.
Increasingly, intelligence is being built into electric grids. Smart grid initiatives seek to improve operations, maintenance and planning by automating operations and ensuring that components of the grid can communicate with each other as required.
See an introduction to the power grid: