  We review the normal and superconducting state properties of the
unconventional triplet superconductor Sr$_2$RuO$_4$ with an emphasis on the
analysis of the magnetic susceptibility and the role played by strong
electronic correlations. In particular, we show that the magnetic activity
arises from the itinerant electrons in the Ru $d$-orbitals and a strong
magnetic anisotropy occurs ($\chi^{+-} < \chi^{zz}$) due to spin-orbit
coupling. The latter results mainly from different values of the $g$-factor for
the transverse and longitudinal components of the spin susceptibility (i.e. the
matrix elements differ). Most importantly, this anisotropy and the presence of
incommensurate antiferromagnetic and ferromagnetic fluctuations have strong
consequences for the symmetry of the superconducting order parameter. In
particular, reviewing spin fluctuation-induced Cooper-pairing scenario in
application to Sr$_2$RuO$_4$ we show how p-wave Cooper-pairing with line nodes
between neighboring RuO$_2$-planes may occur.
  We also discuss the open issues in Sr$_2$RuO$_4$ like the influence of
magnetic and non-magnetic impurities on the superconducting and normal state of
Sr$_2$RuO$_4$. It is clear that the physics of triplet superconductivity in
Sr$_2$RuO$_4$ is still far from being understood completely and remains to be
analyzed more in more detail. It is of interest to apply the theory also to
superconductivity in heavy-fermion systems exhibiting spin fluctuations.
