  We investigate the electronic properties of the tetragonal FeS superconductor
by using scanning tunneling microscope/spectroscopy. It is found that the
typical tunneling spectrum on the top layer of sulfur can be nicely fitted with
an anisotropic s-wave or a combination of two superconducting components in
which one may have a highly anisotropic or nodal like superconducting gap. The
fittings lead to the maximum superconducting gap $\Delta_{max}\approx$
0.90$\;$meV, which yields a ratio of 2$\Delta_{max}/k_BT_c\approx$ 4.65. This
value is larger than that of the predicted value 3.53 by the BCS theory in the
weak coupling limit, indicating a strong coupling superconductivity. Two kinds
of defects are observed on the surface, which can be assigned to the defects on
the S sites (four-fold image) and Fe sites (dumbbell shape). Impurity induced
resonance states are found only for the former defects and stay at zero-bias
energy.
