How to Blur a Background in Photoshop
Blurring the background while leaving an object in the foreground in sharp focus can make a photo pop out to the viewer and highlight the dramatic effect. Photoshop and its use of masks, layers and filters can offer several ways to achieve that effect. Employing the Quick Mask edit mode to mask what you want to stay in focus provides you an easy way to add drama to an image.
Select the Quick Mask edit mode in the tool box by clicking the icon of a circle within a square. You will not see anything different in the image at this point.
Open the Brush tool in the tool box by clicking the "Brush" icon or typing "B." In the Brush Controls menu, select a size large enough to cover broad areas of the part of the photo you wish to stay in focus without overlapping the edges. The cursor turns into a circle for the size of the brush. Click, and the area the brush is on turns red, representing the mask.
Use smaller brushes of even just a few pixels wide to cover finer details along the edges of the red mask you're making. If you mistakenly move the brush too far, erase that part of the mask with the eraser tool.
Click again on the Quick Mask edit mode, or type "Q," and the image returns to standard editing mode. Everything except what you covered in red is selected, shown by a dotted line.
Select "Filter/Blur/Gaussian Blur" to blur the background. You can adjust the radius of the blur to create a desired affect--about one or two pixels should be enough. More and the blur looks unnatural unless you're going for a surreal look.
Press "Control"-"D" (Windows) or "Command"-"D" (Mac) to deselect the background.