Dozens of gravitationally self-bound stellar groupings are known to orbit the halo of the Milky Way. At the faint end, some of these satellites are globular clusters: compact (few pc size), chemically homogeneous systems whose internal dynamics show no evidence for dark matter. Others, on the other hand, are dwarf galaxies: typically larger (~kpc size) systems with varied stellar populations and internal kinematics suggestive of the presence of large amounts of dark matter.  More recently, a number of satellites with properties straddling these two categories have been identified: they are compact in size, but fainter than traditional globular clusters or dwarf galaxies, and with velocity dispersions so low that only upper limits can be placed on their dark matter content. I will discuss the cosmological significance of these systems for the clustering of dark matter in the smallest scales, and for our understanding of the formation of the faintest galaxy systems in the Universe.