The bpy.libraries submodule.
This module provides access to objects stored in .blend files. With
it scripts can append from Blender files to the current scene, like the
File->Append menu entry in Blender does. It allows programmers to
use .blend files as data files for their scripts.
Warning:
This module is new and being considered as a replacement for the original Library
module. Users should stay tuned to see which module is supported
in the end.
Example:
import bpy
scn= bpy.data.scenes.active # get current scene
lib = bpy.libraries.load('//file.blend') # open file.blend
ob = scn.objects.link(lib.objects.append('Cube')) # append Cube object from library to current scene
mat = lib.objects.link('Material') # get a link to a material
me = ob.getData(mesh=1) # get mesh data
me.materials[0] = mat # assign linked material to mesh