A galaxy is a system of stars, dust and gas held together by gravity.  There are many galaxies spread throughout the universe, they range in diameter from a few thousand light years to half a million light years.  A light year is the distance that light travels in one year - about 9.46 trillion kilometres.  Large galaxies have more than a trillion stars, and smaller galaxies have fewer than a billion.
Astronomers do not know how many galaxies there are in the universe, but they believe that there are billions.  Scientists classify galaxies according to appearance and shape.  There are two main types - spiral galaxies and elliptical galaxies.  Most galaxies are so far away that we can not see them without a telescope.