  HERAFitter is an open-source package that provides a framework for the
determination of the parton distribution functions (PDFs) of the proton and for
many different kinds of analyses in Quantum Chromodynamics (QCD). It encodes
results from a wide range of experimental measurements in lepton-proton deep
inelastic scattering and proton-proton (proton-antiproton) collisions at hadron
colliders. These are complemented with a variety of theoretical options for
calculating PDF-dependent cross section predictions corresponding to the
measurements. The framework covers a large number of the existing methods and
schemes used for PDF determination. The data and theoretical predictions are
brought together through numerous methodological options for carrying out PDF
fits and plotting tools to help visualise the results. While primarily based on
the approach of collinear factorisation, HERAFitter also provides facilities
for fits of dipole models and transverse-momentum dependent PDFs. The package
can be used to study the impact of new precise measurements from hadron
colliders. This paper describes the general structure of HERAFitter and its
wide choice of options.
