Backlighting in photography is just what it sounds like, lighting a subject from the back. It can be used for a variety of different ways, such as a hair light for people, showing transparency with fabrics or glass or creating stunning silhouettes. Backlighting is a great way to showcase your subjects, but can lead to some rather poor results if you don't execute it properly.
Using backlighting for portraits is a great way to show some separation between subject and background and lighting the hair from behind always adds a nice touch. Done improperly though, you end up with the subject being dark and under exposed, like this:
This mistake is easily fixed by using a flash for fill light. It will properly expose the face of your subject but still give the rim-light around the head. A great example of natural window light from behind a subject, which is then illuminated by an on-camera flash is:
Backlighting can be used to showcase the transparent nature of certain objects. In this case, you want to expose for the exact item you are shooting and not utilize the flash. If you use the flash as a fill here, you'll wash out the subject and the transparent nature of it won't be apparent. Properly done, you should have an end result like this:
Silhouette's are some of the most breath taking photos you can take, when done properly. The most common mistake with a silhouette is the lack a proper black. There are two key elements that make a successful silhouette, solid black subject matter and properly exposed background. In the photo below of the ship, you can see solid blacks through the entire ship and the sunset is a beautiful, warm color.
There should be one main question you should be asking yourself when you're shooting with backlighting, what is the focus of my subject? That will then tell you whether or not you need to utilize a flash or not. After you've established whether or not you need a flash, you can focus on composition and exposures.
Sometimes, backlighting can happen when you're shooting people, say at an outdoor wedding reception or birthday party and your angle in relation to the subject and the sun's current position will be changing by the minute. When this happens, have your flash ready and pay attention to shadows on the ground. If a person's shadow is coming towards you, use your flash as a fill, it should be something you can turn on and off with your eyes closed.
With some careful thinking prior to shooting, you can avoid most all common backlighting mistakes. A few seconds of preparation will make a lasting impression.