Have you ever taken a picture of a landscape scene and had the sky over-exposed, or the foreground under-exposed? This is because you camera's sensor dynamic range is limited. There are several methods to combat this:
The first method involves using Photoshop, the second uses an easy-to-use piece of software called Photomatix to do it automatically.
For the Photoshop method click here
For using the Photomatix software method click here