A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 workThe Realm of the Nebulae[1] and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside inglobular clusters.Video: http://videos.howstuffworks.com/nasa/13490-ask-an-astronomer-spiral-galaxies-video.htm
