In the late 1700's Charles Messier published a list of objects
in the sky that were definitely not stars or comets. Using a weak 4-inch refracting telescope, his goal was to note permanent, cloudy objects that were not visible from northern latitudes in order to assist people in finding comets, the mysterious cloudy entities that flew across the sky and changed position from one night to the next. The 110 entities he found ended up being the nearest and brightest of five categories of objects that we today call 'Deep Sky Objects
' - diffuse nebulae, planetary nebulae, open clusters, globular clusters, and galaxies - that were visible from the Northern Hemisphere. His designations are still used for what became a very famous list of objects, and amateur astronomers sometimes hold "Messier Marathons" where they try to spot all of these relatively easy-to-see objects in one night. This is what we're calling on when we refer to one of the brightest shapes in the sky as "M31 - Andromeda", and these tend to be the easiest visually interesting things to image outside our solar system.
Here is a nice poster
of the Messier list in natural color, as well as a more detailed gallery
, courtesy of Alistair Symon