Globular clusters are ancent collections of stars ranging in age that are gravitationally bound together in a spheroidal shape. Some are almost as old as the universe itself. They can contain anywhere from tens of thousands of stars to millions of stars. Globular clusters can range in size between a few dozen and a few hundred light years. Almost all larger galaxies have globular clusters in orbit around them and the Milky Way is no exception with around 200 of them circling its periphery. The much larger galaxy Messier 87 has as many as 15,00 globular clusters gracing it with their presence. 

It's unclear how globular clusters formed in the early history of the universe. Some conjecure that they may have been failed galaxies or small galaxies that were stripped of most of their content by larger ones.