Why are galaxies disk shaped? I always see galaxies in "disk" shape in pictures. It is like, there is a rotational movement on a plane and the system is balanced by the pulling-in gravitational force which prevents the stars to leap out of the galaxy. What is the physical reason behind this? I have never seen a star in disk shape. A star is nothing but a much much smaller non-solid mass revolving around an axis, just like the galaxies. Does size matter? What makes the difference? Why doesn't a typical galaxy form in a spherical shape rather than a disk shape?
Spiral galaxies may be the most commonly depicted probably because we find ourselves in one, however, galaxies come in many sizes and shapes. Wikipedia's page on Galaxy morphological classification explains the most common classifications. 

Answering the question on the title:


  The basic answer to your question is Conservation of Angular Momentum.
  Astronomers have been interested in this question for a long time and
  are still writing papers on the subject.   Even if you assume that at
  the time of the big bang there was no net angular momentum in the
  universe, one would expect that there were local fluctuations.  When
  galaxies initially formed by gravitational attraction these net local
  amounts remained.  Now angular momentum is conserved so when the
  initial collection of stars and gas in one galactic volume start to
  collapse the net angular momentum has to stay the same.  Since angular
  momentum $\vec{L} =m\vec{v}\times\vec{r}$,  for a given $v$ its easier to conserve $L$ if the $r$ is
  large.   In the orthogonal direction (along the axis of the accidental
  angular momentum) there is no such hindrance to collapse and so the
  collection turns out to be disk shaped.   By the way there has been
  several studies on the net angular momentum of a large collection of
  galaxies coming from the Sloan Digital Sky Survey and the answer is
  pretty close to but not quite zero.  Stay tuned.


Extracted from an University of Phoenix Department of Physics article