A few years back when doing research on the mass of dwarf galaxies I asked my advisor what the difference was between a globular cluster and a dwarf galaxy. I was more-or-less scoffed at, as the "obvious" textbook answer is that globulars consist of only old stars and don't have any gas, while galaxies of any sort will have stars of varying ages and will have gas.
New research using Hubble and Gemini South shows the question isn't quite as clear-cut as textbooks like to put it. Our Milky Way galaxy has all these globular clusters orbiting it, but we've also got some dwarf galaxies like the Large and Small Magellenic Clouds. There's this one strange globular cluster, Omega Centauri, which has got multiple generations of stars and a large mass (causing its fast spin), but doesn't seem to have any gas. And now they've discovered that there's an intermediate mass black hole in the center, implying that that Omega Centauri is probably a dwarf galaxy, not a globular cluster after all. Huh, go figure.