>HeavyJet mentioned that it is an aft belly cargo door. He's correct, however the MAIN cargo doors are located on the right side of the 767 (just in front and just behind the right wing). An L-1011 has the same Large cargo doors on the starboard side.
>HAWK21M, Here's a pix of the RIGHT side of the same 767 that you posted.
Click for large version
Photo © Y.K. Cummins NG