A grid field is an array of geostationary satellites which generate a magnetic field around a planet. They typically have gaps in their coverage to allow ships to have access to space.
The grid satellites are composed of a central module, typically a meter across, surrounded by rings spaced about a meter apart. Four central support beams stretching about a 3.5 kilometers long. These rings have an electric current running through them resulting in a weak magnetic field. This magnetic field is about 50 microteslas (virtually equal to that of Earth's). The satellites are spaced and layered so that few places are without coverage.
An interesting thing that occurs with these satellites are the micro-aurora that occur near the center module.
List of planets with a grid field
(This list is incomplete. You can help by expanding it)