  We describe in this paper the ROSAT Ultra Deep Survey (UDS), an extension of
the ROSAT Deep Survey (RDS) in the Lockman Hole. The UDS reaches a flux level
of 1.2 x 10E-15 erg/cm2/s in 0.5-2.0 keV energy band, a level ~4.6 times
fainter than the RDS. We present nearly complete spectroscopic identifications
(90%) of the sample of 94 X-ray sources based on low-resolution Keck spectra.
The majority of the sources (57) are broad emission line AGNs (type I), whereas
a further 13 AGNs show only narrow emission lines or broad Balmer emission
lines with a large Balmer decrement (type II AGNs) indicating significant
optical absorption. The second most abundant class of objects (10) are groups
and clusters of galaxies (~11%). Further we found five galactic stars and one
''normal'' emission line galaxy. Eight X-ray sources remain spectroscopically
unidentified. The photometric redshift determination indicates in three out of
the eight sources the presence of an obscured AGN in the range of 1.2 < z <
2.7. These objects could belong to the long-sought population of type 2 QSOs,
which are predicted by the AGN synthesis models of the X-ray background.
Finally, we discuss the optical and soft X-ray properties of the type I AGN,
type II AGN, and groups and clusters of galaxies, and the implications to the
X-ray backround.
