Here's an extreme example:
Foggy morning at Radnor Lake .....
Look at the histogram for this photo and you'll see that both the white and black points, the points on the bottom of the histogram where the data from the image stops, are well away from the ends of the histogram. That's because there are no true black or white values in this photograph.
Absolutely no changes whatsoever other than dragging the black point to the right and the white point to the left. This stretches out everything in the middle, gives it a true black point and a true white point, and the fog is virtually gone. The trees in the background that were shrouded in fog are now relatively clear.
DISCLAIMER!!!!!!
Before people start telling me everything that's wrong with the second image I AM AWARE OF IT. I would not recommend this as a finished image, I posted is purely and simply to show what can be done in about 15 seconds to eliminate fog and haze. It needs cropping, it needs composition help (I couldn't even see the bank of the lake when I took it and it's dead center of the image now), it needs contrast help, it needs everything. I know that already, it was just an example.