All of the brighter star clusters, nebulas, and galaxies are assigned numbers in deep-sky catalogs. Be aware that many objects have two designations: the Great Nebula in Orion, for example, is known as M42 and also as NGC 1976.
Catalog numbers can help avoid the confusion that arises with common names. There are three objects sometimes known as the Pinwheel Galaxy. If you wish to specify the one in Ursa Major, you can call it M 101 (or NGC 5457), and astronomers around the world will know which one you mean.