From Diablo Wiki
The Horadric Cube is a powerful artefact made by the Horadrim brotherhood, capable of magically merging or transforming objects inside it. The box outer size is significantly smaller than the inside.
The Horadric cube features prominently in Diablo II, and you can read more about the .
 The Cube in Diablo III
The Horodric Cube is not in diablo 3.