A quasi-coherent sheaf on a scheme 
is a sheaf 
of 
-modules such that, on every open subscheme 
that is an affine scheme,
its restriction has the form 
for an 
-module 
.
The association 
translates algebraic constructions on modules into geometric
constructions on sheaves. In particular, quasi-coherent
ideal sheaves correspond to closed
subschemes.
