Charles Messier (1730–1817) was a French astronomer who had a deep interest in comets. But as he scanned the night sky, he was often cataloguing things that could be mistaken for a comet. In order to eliminate these objects, Messier created a catalogue of these objects, creating the Messier Catalogue, familiar to all astronomy enthusiasts. Later we learned the true nature of the objects Messier described. We would call them deep sky objects now.
Deep sky is everything beyond our solar system. The stars, their groupings in clusters, the nebulas they are born in, the nebulas left behind when stars die, the Milky Way and the galaxies beyond. There is a vast zoo of objects to explore, all within the reach of amateur instruments!