  A geometric stack is a quasi-compact and semi-separated algebraic stack. We
prove that the quasi-coherent sheaves on the small flat topology, Cartesian
presheaves on the underlying category, and comodules over a Hopf algebroid
associated to a presentation of a geometric stack are equivalent categories. As
a consequence, we show that the category of quasi-coherent sheaves on a
geometric stack is a Grothendieck category.
  We associate, in a 2-functorial way, to a 1-morphism of geometric stacks $f$,
an adjunction $(f^*, f_*)$ for the corresponding categories of quasi-coherent
sheaves that agrees with the classical one defined for schemes. This
construction is described both geometrically in terms of the small flat site
and algebraically in terms of the Hopf algebroid.
