What is flat light and how to counter that?
Flat lighting is when a scene, regardless of its type, is largely and broadly "diffusely and directly lit". Flat lighting illuminates the scene, however it does not bring out depth and detail, or add any intriguing character. In some rarer/specific cases, you may require flat lighting, depending on the exact goals you have. In general, however, flat lighting makes a photo look rather dull and lifeless.
With nature photography, flat lighting is often caused by overcast skies, direct sunlight, etc. Landscape photos in particular generally look best when the sun is at a low, indirect angle such that the shape of mountains cast shadows upon themselves, bringing to light that shape and providing depth. Early morning sunrise or evening sunset light is good for this reason, as well as for the fact that such light is often more colorful and vibrant than midday sunlight.
With portrait photography, flat lighting dulls or eliminates the intriguing features of your subjects that give them their character, expression, emotion. Highly diffused, direct lighting can be a useful tool for portraits, however uses as the only source of lighting it can produce rather boring portraits. More complex, angled lighting can do wonders for portrait photography. Use of sunlight or artificial lighting, angling your subjects to light sources, providing adequate and appropriate fill lighting, etc. can greatly enhance the character and emotion of your subjects. I would search our forum for other questions of portrait and studio lighting...there are many skilled photographers here who have provided superb answers detailing specific kinds of lighting setups for portrait photography that go into far greater depth than I ever could here.
Flat is not necessarily diffused. A built in flash creates a flat image but is anything but diffused.
@ysap - And now we have touched on the two sides of the argument! "Flat" light has many definitions that I have commonly heard!
@ysap: True...and diffuse is not exactly the word I am looking for. But neither is direct...as flash lighting IS normally diffused a bit by the "lens" that covers the bulb.