Charles Messier was a comet hunter in the mid to late 1700s. You may be familiar with the pictures of comets that look like stars with long tails. This is what they look like then they are close to the sun. When they are first approaching the solar system, they look like fuzzy patches in the telescope, like faint balls of cotton.
There are many objects in our galaxy that look similar to comets that are not. Messier knew this since they do not move relative to the stars. In order not to confuse some of these objects with a possible comet, Messier cataloged these fuzzy objects and gave them numbers.
As telescope technology improved, astronomers realized that these fuzzy objects were nebulae, star clusters, and galaxies. Today we use this catalog as a map of cool things to look at. There are 101 objects in the catalog. Many astronomy clubs have contests to see how many of the objects their members can find. In they spring they hold Messier Marathons which are all night observing sessions. It is actually possible in the spring to view all of the objects in one night.