This effect happens all the time!
The picture has been taken indoors so the camera will have set itself to use a long exposure in order to take the picture (i.e. it's recording the image for 1/20s or something like that). Whilst the picture was taken someone was walking down the stairs, their body was moving hence it blurred in the shot, but the hand remained still on the banister which is why you can see some detail in it.
For example see the following picture taken from
http://www.photoaxe.com/motion-blur-panning-and-zooming-tricks-in-photography/
As you can see the people walking/moving are blurred, but the people who are not moving/sat down are not blurred. The faster someone is moving the more they will appear to blur (when camera settings remain consistent!!!)