Level[expr, levelspec] gives a list of all subexpressions of expr on levels specified by levelspec.
Level[expr, levelspec, f] applies f to the list of subexpressions.
Level uses the standard level specification described in Section A.3.6.
Level[expr, -1] gives a list of all "atomic" objects in expr.
Level traverses expressions in depth-first order, so that the subexpressions in the final list are ordered lexicographically by their indices.
See The Mathematica Book: Section 2.1.7.
See also: Apply, Map, Scan.