  I review the basics of the collinear factorization theorem applied primarily
to deep inelastic scattering (DIS) involving forward parton distributions
(PDFs) and the extensions of this theorem for exclusive processes probing
non-forward parton distributions (GPDs), the generalized distribution
amplitudes (GDAs) and the transition distribution amplitudes (TDAs). These QCD
factorization theorem is an important tool in the description of hard processes
in QCD. Whenever valid, it permits to represent the cross section or the
scattering amplitude for such a process as a convolution in partonic momenta of
a perturbatively calculable part (the coefficient function, CF) which involves
the hard scale of the process with non-perturbative (soft) distributions of
active partons inside the hadrons involved in a process. The reliability of the
perturbatively determined hard part together with high precision experimental
data on relevant observables gives a hope for the possibility to uncover fine
details of interpartonic interactions and spatial distributions of partons
inside hadrons. I conclude with some remarks about QCD factorization with
transverse momentum dependent PDFs
