The word galaxy is derived from the Greek galaxias literally "milky", a reference to the Milky Way.
A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component called dark matter.
Galaxies contain varying amounts of star systems, star clusters and types of interstellar clouds.
The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical.
Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars.
Despite the prominence of large elliptical and spiral galaxies, most galaxies in the Universe appear to be dwarf galaxies. These galaxies are relatively small when compared with other galactic formations.