  These lectures include an introduction to the partonic description of the
proton, the photon and the `colour singlet', as seen in inclusive and
semi-inclusive DIS, in $e^+e^-$ collisions, and in diffractive processes,
respectively. Their formal treatment using structure, fragmentation, and
fracture functions is outlined giving an insight into the perturbative QCD
framework for these functions. Examples and comparisons with experimental data
from LEP, HERA, and Tevatron are also covered.
