  Ultraluminous X-ray sources (ULXs) are accreting black holes that may contain
the missing population of intermediate mass black holes or reflect
super-Eddington accretion physics. Ten years of Chandra and XMM-Newton
observations of ULXs, integrated by multiband studies of their counterparts,
have produced a wealth of observational data and phenomenological
classifications. We review the properties of their host galaxies, list popular
spectral models and implications for standard and supercritical accretion
physics, demonstrate how X-ray timing of these objects places constraints on
their masses. We also review multiwavelength studies of ULXs, including the
optical emission of the binary system and nebulosity around them. We summarize
that three classes of black holes could power ULXs: normal stellar mass black
holes (~10 solar masses), massive stellar black holes (~< 100 solar masses),
and intermediate mass black holes (10^2 - 10^4 solar masses). We collect
evidence for the presence of these three types of compact objects, including
caveat of each interpretation, and briefly review their formation processes.
