depth of field

In optics, particularly as it relates to film and photography, the optical phenomenon known as depth of field , is the distance about the plane of focus (POF) where objects appear acceptably sharp in an image. Although an optical imaging system can precisely focus on only one plane at a time, the decrease in sharpness is gradual on each side of the POF, so that within the DOF the unsharpness is imperceptible under normal viewing conditions.

  The first photo was taken in July this  year in Chicago, Illinoils. As you can appreciate, this guy is focused, whereas the Chicago skyline is blurred.

 In the second one, you see another guy focused and colorful blurred background.