  Structure functions are a measure of the partonic structure of hadrons, which
is important for any process which involves colliding hadrons. They are a key
ingredient for deriving partons distributions in nucleons. In recent years
dramatic progress has been made in the understanding of the nucleon structure
and the precision of its partonic content, due to vast theoretical progress,
and the availability of new high precision measurements. This review gives an
overview on present structure function and related data, and on the most recent
techniques used to extract parton distribution functions to describe the
structure of the proton. Special attention is given to the determination of the
uncertainties on the parton distributions.
