I will review the properties of ultra-compact dwarf galaxies (UCDs), a class of stellar systems covering the parameter space between omega Centauri and M32. They are believed to be either the most massive star clusters in the universe, or tidally truncated galaxies, or, both. In the talk I will focus mostly on two aspects: 1. The specific frequencies of UCDs - a recently introduced quantity that allows to test whether the luminosity distribution of UCDs follows the bright tail of the globular cluster luminosity function. 2. The elevated dynamical M/L ratios of UCDs. Are they due to a variation of the IMF, or, due to supermassive central black holes as relics of their galaxian nature, or, dark matter? In this context I report on recently started adaptive optics spectroscopy campaigns targeting UCDs in which we aim at resolving their internal dynamics. I will conclude the talk with a discussion on whether the biggest star clusters in the Milky Way are in reality galaxies.