To make a good photo, you not only have to look at your subject, but also at the space that surrounds it. Both foreground, background and how they relate to each other are important factors that can either help focusing your attention on the main subject or distracting from it.
The space that surrounds your subject is made up of two parts, the foreground and the background.
When you look at a scene with both your eyes, you see a 3D image. This makes it easier to separate subjects from their surroundings. A camera however has only one eye and you will end up with a 2D image. Or, to put it in another way, the 3D image you see will be flattened to a 2D surface, being a photo or a computer screen. This makes it very important to incorporate the background into the composition. Even when you blur the background, the least you will see will be some shapes and colours.
There are several ways in which a background can add to your photos, like
In all cases you have to make sure the background does not have too many distracting elements in it, since these will pull the attention away from the main subject.
Also, keep an eye out for elements in the foreground that can act as natural frames; they can add a real sense of perspective.
Here’s a little exercise you can do to see how your subject interacts with its background:
Depth of field preview, if your camera has it, can give you an idea of how much of your fore/background will be in acceptable focus.
For more information on this topic, you can also check out following TFTTF shows: