By manipulating the depth of field using the proper combination of aperture, shutter speed, and focal length, you can capture an image where the subject really stands out by causing the background to be out of focus.  This is called selective focus.

Wildlife and sports photographers use the technique of selective focus often.  If wildlife photographers did not blur the background of an image of bird and left the leaves and branches in focus, the bird would lost.  Similarly, with all the action on a football field, and the people in the stadium, selective focus is used to make the athlete stand out in a very cluttered and colorful environment.

There are photo editing techniques you can learn to blur the background, simulating selective focus.

First you would select the subject of your photo using one of the selection tools of your image editing program, like the “Lasso” tool in Photoshop.  Then “Invert” your selection so that the background is now selected.  You will also want to work with the “Feathering” option to optimize how soft or sharp the edge of your subject is.  Feathering is a matter of personal taste.

Next you would use the “Gaussian Blur” filter.  Gaussian Blur is an algorithm designed to mimic the blurring caused by selective focus.  Try different settings in this filter until you get the effect you’re looking for.  Often it is more pleasing to the eye to blur the background so that it recognizable but out focus.

Panning is a related blurring technique.  Panning is achieved by tracking a moving subject with the camera while you take the picture.  Done properly the subject is in focus and the background is blurred horizontally.  Panning gives the image a sense of motion too.

Your photo editing software calls this “Motion Blur”, or “Radial Blur”.  By using radial blur your subject will appear to be moving across the background.

Photo Credits:
Bird with Blurred Background, flickr.com/gusjer
Panning Cyclist, flickr.com/zoutedrop