The tail cone is where the exhaust is for the APU (auxiliary power unit), or the "third engine" although it's not used to fly the plane, it is there to run the electrical systems and such in the event of a problem. It's past the vertical and horizontal stabilizers of the tail, the butt of the plane

Where that exhaust opening is, that's what I call the tail cone. The 777's cone tapers into a fin shape, while the 767s tail cone actually is a conical shape. If you go to airliners.net, you'll see what I mean. It's actually how I can tell the DC-9 from the MD-8* series, they have the same thing. The DC-9 is conical in shape, and the MD-8* series are fin shaped.

Yes, I am an airplane geek - my Husband gives me crap all the time