T-Bone steaks and Porterhouse steaks are the same. The Porterhouse is just a larger version of the T-Bone because it is carved from the larger portion of the tenderloin.

Both contain a bone that is in the shape of a "T". On each side of the bone is a valued cut. In the case of the "T" bone, one side, when filleted is a New York Strip and the other is simply a "filet". (from which we get Filet Mignon) 

The difference between a t-bone and a porterhouse is in the size of the filet. The very thick, tenderer filet we call Mignon is traditionally from the porterhouse cut.