Key issues of classical and quantum strings in gravitational plane waves,
shock waves and spacetime singularities are synthetically understood. This
includes the string mass and mode number excitations, energy-momentum tensor,
scattering amplitudes, vaccum polarization and wave-string polarization effect.
The role of the real pole singularities characteristic of the tree level string
spectrum (real mass resonances) and that of spacetime singularities is clearly
exhibited. This throws light on the issue of singularities in string theory
which can be thus classified and fully physically characterized in two
different sets: strong singularities (poles of order equal or larger than 2,
and black holes), where the string motion is collective and non oscillating in
time, outgoing and scattering states do not appear, the string does not cross
the singularities, and weak singularities (poles of order smaller than 2, Dirac
delta, and conic/orbifold singularities) where the whole string motion is
oscillatory in time, outgoing and scattering states exist, and the string
crosses the singularities. Commom features of strings in singular plane
backgrounds and in inflationary backgrounds are explicitly exhibited. The
string dynamics and the scattering/excitation through the singularities
(whatever their kind: strong or weak) is fully physically consistent and
meaningful.