There most certainly is a difference and as far as I know this is it. The ATB consists of 2 separate units. ie Tug and Barge, the latter having a “notch” in the stern into which the tug is fitted. This combination can and often is separated for various reasons on occasion, and the tug can actually “Tow” the Barge if needed. Example heavy seas, on a long tow. But more importantly it retains some flexibility in the coupling.
The ITB on the other hand is connected rigidly to the Barge, usually with a hydraulic coupling which essentially makes the 2 units into one rigid “Ship”. The term Integrated implies that neither unit can operate without the other.
Just found this which more or less says same thing. http://www.crowley.com/petroleum-chemical-transportation/atb-whatis.asp