Objects near the flash will be linghetr than object farther away. You can use this effect to isolate a key figure against a less distracting background. When photographing people, you'll often see images with what's called red eye. The light from a flash has entered through the subject's pupil and reflected off the retina at the back of the eye and then back out to the camera. Since the retina is full of thin blood vessels, it takes on a red color.
Some flash units have a red-eye reduction mode.
These work by firing a separate red-eye reduction lamp, or a series of short flash bursts from the main flash to close the iris a little before firing the actual flash used to take the picture. Eric demonstrates one way to get rid of redeye. It's not just humans you can have problems with. Here's a shot of an owl with a bad case of redeye. I shot it out of doors using flash and never expected this.