  In this survey paper (which supersedes our earlier arXiv preprint
math.AG/0507086) we give a relatively simple and coordinate free description of
the WZW model as a local system whose base is a G_m-bundle on the moduli stack
of pointed curves. We derive its main properties and show how it leads to a
modular functor in the spirit of Graeme Segal (except for unitarity). The
approach presented here is almost purely algebro-geometric in character; it
avoids the Boson-Fermion correspondence, operator product expansions as well as
Teichmueller theory.
