In the late 18th century, the French comet hunter Charles Messier became frustrated when he kept sighting faint, diffuse celestial objects he mistook for comets. To prevent confusion, he cataloged the positions of 103 of these objects. Armed only with a tiny telescope, Messier had no idea what these objects were. But more than 200 years later, his catalog is now the most well-known list of galaxies, stars clusters, and nebulae accessible with small telescopes.
Objects in the Messier list are designated with an M and a number. The Crab Nebula in Taurus, for example, is M1. The Pleiades is listed as M45. And the lovely “Wild Duck” star cluster in Scutum is M11. Since Messier was based in the northern hemisphere, all the objects in the catalog are in the northern and near-southern sky, though many of the objects can be seen in populated areas of the southern hemisphere. Messier’s catalog was later expanded to 110 objects. With dark sky, you can see all 110 objects in a 3” or 4” telescope, though usually not all in one night. (Although there is a window in March when observers in some parts of the world can see all the Messier objects in one night… an astronomical endurance event known as a “Messier Marathon”).
Of course, there are far more than 110 sights to see in the night sky, and later astronomers compiled more extensive catalogs. J. Dreyer developed the New General Catalog (NGC), which contains the positions of 7,840 objects observed by William Herschel and others. Like the Messier catalog, the NGC lists galaxies, star clusters, and nebulae. Unlike the Messier catalog, not all objects are accessible with backyard scopes, though you can see hundreds or thousands of NGC objects, depending on your skies, your skill, and your telescope.
There are other catalogs, including the Index Catalogs (IC), which extend the NGC, the Collinder and Melotte catalogs of open star clusters, and E. E. Barnard’s catalog of dark nebulae. You will come across objects in these catalogs from time to time in your exploration of the night sky.