  The TACTIC $\gamma$-ray telescope, equipped with a light collector of area
$\sim$9.5m$^2$ and a medium resolution imaging camera of 349-pixels, has been
in operation at Mt.Abu, India since 2001. This paper describes the main
features of its various subsystems and its overall performance with regard to
(a) tracking accuracy of its 2-axes drive system, (b) spot size of the light
collector, (c) back-end signal processing electronics and topological trigger
generation scheme, (d) data acquisition and control system and (e) relative and
absolute gain calibration methodology. Using a trigger field of view of
11$\times$11 pixels ($\sim$ 3.4$^\circ$$\times3.4^\circ$), the telescope
records a cosmic ray event rate of $\sim$2.5 Hz at a typical zenith angle of
15$^\circ$. Monte Carlo simulation results are also presented in the paper for
comparing the expected performance of the telescope with actual observational
results. The consistent detection of a steady signal from the Crab Nebula above
$\sim$1.2 TeV energy, at a sensitivity level of $\sim5.0\sigma$ in $\sim$25 h,
alongwith excellent matching of its energy spectrum with that obtained by other
groups, reassures that the performance of the TACTIC telescope is quite stable
and reliable. Furthermore, encouraged by the detection of strong $\gamma$-ray
signals from Mrk 501 (during 1997 and 2006 observations) and Mrk 421 (during
2001 and 2005-2006 observations), we believe that there is considerable scope
for the TACTIC telescope to monitor similar TeV $\gamma$-ray emission activity
from other active galactic nuclei on a long term basis.
