Exposure values or EVs are numbers that refer to combinations of lens aperture and shutter speed. Your eye is capable of discerning 12EV but a typical camera can only handle about 6.
So if you shoot the same image at a range of speeds and sandwich them together you can see a higher dynamic range, one closer to what you're used to seeing with your eye. Photoshop has a handy tool that will help combine multiple images, and there are several stand-alone programs to help too.
Here's a shot properly exposed for the tree trunk (click any image to enlarge):
There's 4.0EV difference between the black, dark tree trunk and the bright, white surf and clouds behind. Here's the shot combined with a properly exposed beach (Kauai HI).
There are a few artifacts that give away the fact that more than one images is involved: note the branches at the top that were moving in the tradewinds. If there are moving people or cars in one of the shots they'll look ghostly.
This picture, taken just after sunrise, is made up of 4 images with a range of 6.5 EV. When the sky was properly exposed the mountain was just a black silhouette. Properly exposed for the mountain, the sky was completely blown out, totally white.
Here's a better example I made a while back in a dark hangar.
But take a look at these and these!