In amateur astronomy, the term deep sky is used to summarize those optically observable celestial objects that are outside the solar system (i.e. not asteroids, planets or comets), but are not individual stars. They include all star clusters, nebulae and galaxies.
Because of the little light the stars give off, many pictures have to be taken for a single picture - the little light of the stars has to be collected in many pictures. These individual shots are then combined (stacked) on the computer with special software.