DownValues[f] gives a list of transformation rules corresponding to all downvalues defined for the symbol f.
You can specify the downvalues for f by making an assignment of the form DownValues[f] = list.
The list returned by DownValues has elements of the form HoldPattern[lhs] :> rhs.
See The Mathematica Book: Section 2.4.13.
See also: Set, UpValues.