Did you know your eyes are able to see a much wider range of light and dark values than your camera can record?
When faced with wildly differing light and dark values in a single frame, your camera can't record everything, so it has to make a choice. Usually, it does its best to meter for the average light in the scene. That means the bright stuff ends up being too bright or the dark stuff too dark... or a little of both. Those dark areas aren't lost, however. Often, they're hiding detail that the camera just barely saw. Make a few subtle adjustments in Photoshop, and you're on your way to an improved image!
Website Photojojo.com has a good article and short video that teaches us how to-
If you're looking to improve your photos, this topic is 'need to know' stuff.