The Messier objects are a set of astronomical objects first listed by French astronomer Charles Messier in his "Catalogue des Nébuleuses et des Amas d'Étoiles" ("Catalogue of Nebulae and Star Clusters") included in the Connaissance des Temps for 1774 (published in 1771). The original motivation of the catalogue was that Messier was a comet hunter, and was frustrated by objects which resembled but were not comets. He therefore compiled a list of these objects, in collaboration with his assistant Pierre Méchain.
The first edition covered 45 objects numbered M1 to M45. The total list published by Messier finally contained 103 objects, but the list "got an independent life" by successive additions by other astronomers, motivating the additions by side notes in Messier's and Mechain's texts indicating that either of them knew of the objects. The first such addition came from Camille Flammarion in 1921, who added Messier 104 after finding Messiers side note in his 1781 edition exemplar of the catalogue. M105 to M107 were added by Helen Sawyer Hogg in 1947, M108 and M109 by Owen Gingerich in 1960, and M110 by Kenneth Glyn Jones. M102 was observed by Méchain, who communicated his notes to Messier; later, it was admitted by Méchain himself that this object does not exist, and it was simply a re-observation of M101. Some sources mention the galaxy NGC 5866 as an identification for M102, but its description does not fit with Méchain's notes.Messier's final catalogue was included in the Connaissance des Temps for 1784 (published in 1781). These objects are still known by their Messier number from this list.
Text & tables courtesy of Wikipedia
Because Messier lived and did his astronomical work in France in the Northern Hemisphere, the list he compiled contains only objects from the north celestial pole to a celestial latitude of about 35.7°. Many impressive Southern objects, such as the Large and Small Magellanic Clouds are excluded from the list. Because all of the Messier objects are visible with binoculars or small telescopes (under favorable conditions), they are popular viewing objects for amateur astronomers. In early spring, astronomers sometimes gather for "Messier marathons", when all of the objects can be viewed over a single night.A summary of the astrophysics of each Messier object, as well as all of the objects in the Herschel 400 Catalog, can be found in Concise Catalog of Deep-sky Objects: Astrophysical Information for 500 Galaxies, Clusters and Nebulae by W. H. Finlay, ISBN 1-85233-691-9.