  (abridged) The review describes observations of SS433. The main difference
between SS433 and other X-ray binaries is the supercritical regime for the gas
accretion onto the relativistic star (most likely a black hole), which has lead
to the formation of collimated relativistic jets. The properties of the jets
are determined by their interaction with the disk wind. The precession of the
disk and jets, as well as the eclipsing in the binary system, make SS433 a
unique laboratory for studies of mechanisms for the microquasar phenomenon.
Essentially all photometric and spectroscopic properties of SS433 are
determined by the accretion disk and its orientation, but the disk itself is
not observed, being located beneath the photosphere of the dense wind.
Observational manifestations of the wind and of gas flows in the system are
described. Little is known about the structure of the central regions where the
hot bases of the jets are located. The available X-ray, UV, and optical
observations point towards the picture where the bases of the jets are
surrounded by cocoons of hot gas reradiating emission from the inner regions of
the jet channel. Direct investigations of this channel in the supercritical
accretion disk of SS433 are not possible; however, a similar object oriented
face-on would likely be an extremely bright X-ray source, such as
ultra-luminous X-ray sources observed in other galaxies.

Astrophysics