The Messier Objects
Messier objects are stellar objects, classified by astronomer Charles Messier in the 18th century, ranging from distant galaxies to star clusters to stellar nebula. The catalog was a major milestone in the history of astronomy, as it was the first comprehensive list of deep sky objects. Ironically, Charles Messier wasn't all that interested in the objects in his list. He made the catalog in order to avoid mistaking those objects for comets, which were his true passion.
Messier objects are identified by Messier Numbers. The first object in Messier's catalog, the Crab Nebula, is labeled M1. The last object, M110, is a satellite galaxy located in the constellation Andromeda. There is no systematic ordering in the Messier Catalog. Messier entered objects into the list as he found them. Sometimes he made mistakes and once he entered the same stellar object twice. The catalog has undergone some slight revisions since Messier's time, correcting the mistakes in the original.
One of the great pursuits for amateur astronomers is to do a Messier Marathon, trying to view all of the objects in Messier's catalog in one night. Unfortunately, if you want to see all of them, you have to start looking right after sunset and continue until just before sunrise - hence the term, "marathon." March is the only month in the year in which an astronomer can run the complete marathon.
You can learn more about the Messier Marathon at Wikipedia and Stargazer's Online Guide.