  A large number of Ultra-Luminous X-ray Sources in external galaxies is now
known, but the discussion about their nature is still unsettled:
intermediate-mass black holes or stellar-mass black holes accreting above the
Eddington limit? A promising path that can provide an answer is through
comparison with "normal" stellar-mass black holes. These display separate
states, identified from both their energy spectra and their fast variability.
Although the nature of these states and their transitions is not clear, such a
comparison can be extremely useful. In this paper, I briefly outline our
current phenomenological knowledge of these states and present a (partial)
overview of the different approaches followed to make these comparisons.
