Well, a VOB is a Vue OBject, and an OBJ can be an Imagine OBJect or (more familiarly) a Wavefront OBJect, and a 3DS is a 3D Studio object. They are all 3D model object formats, but each format is different. VOB can only be opened by Vue. Wavefront OBJ and 3DS can be opened by a variety of programs. :)