FGD

An FGD file allows Hammer to read information about entities such as keyvalues, inputs, outputs, and flags. The letters mean Forge Game Data, The Forge being the name of Hammer before it was released. An FGD is solely used by Hammer. FGD data is not required in any way in order to make a map, though they do make the process much easier. It should also be noted that FGDs can be incomplete or inaccurate, as Valve's tend to be. Modifying an FGD doesn't actually change any of the entity's functionality.

For info on editing an FGD, see Parts of an FGD.