Text and description quoted verbatim from Microsoft DirectX 7 help file, included here for convenient reference.

The D3DFOGMODE enumerated type contains constants describing the fog mode. These values are used by the D3DRENDERSTATE_FOGTABLEMODE and D3DRENDERSTATE_FOGVERTEXMODE render state in the D3DRENDERSTATETYPE enumerated type.

typedef enum _D3DFOGMODE {
D3DFOG_NONE = 0,
D3DFOG_EXP = 1,
D3DFOG_EXP2 = 2,
D3DFOG_LINEAR = 3
D3DFOG_FORCE_DWORD = 0x7fffffff,
} D3DFOGMODE;

D3DFOG_NONE
No fog effect.
D3DFOG_EXP
The fog effect intensifies exponentially, according to the following formula:
f=1/e^d*density

D3DFOG_EXP2
The fog effect intensifies exponentially with the square of the distance, according to the following formula:
f=1/e^(d*density)^2

D3DFOG_LINEAR
The fog effect intensifies linearly between the start and end points, according to the following formula:
f= (end - d) / (end - start)

This is the only fog mode currently supported.

D3DFOG_FORCE_DWORD
Forces this enumeration to compile to 32 bits in size. This value is not used.
Remarks
In monochromatic (ramp) lighting mode, fog works properly only when the fog color is black. (If there is no lighting, any fog color works since, in this case, any fog color is effectively black.)

For more information, see Fog.

Note Fog can be considered a measure of visibility - the lower the fog value produced by one of the fog equations, the less visible an object is.