As jhocking said in the comment, FOW is unknown areas.
Computers can't draw infinite terrain. The lag would be incredible. Fog is used to blend in the edge of the render distance, which doesn't need to be close to the camera, nor does there need to be a large blend range.
You sound like you are thinking of a very short view range. Increasing your view range to a large number (this is set in your projection matrix), and sticking fog at the end, makes it seem like an endlessly shown terrain. And the fog isn't really that noticable.
However if you still want a precise answer to your question,
Weather, such as rain, snow, wind, etc. can help obscure the out of range area.
Terrain which reaches the clouds... Having mountains in your terrain, then having clouds at the top of the world, means that in the distance, you will find the edge of the world hidden by mountains, (or if your high up, clouds).
Finally, if your really desperate you could stick a curve onto your world, so that it appears that things in the difference are disappearing over the horizon.
However i still suggest you use fog, as it is simply a blending for the out of range area.