A Porterhouse steak is basically just a larger version of a T-bone steak. It is a thicker cut of meat that has more tenderloin compared to the loin portion. It is the perfect sized steak for two people since it comprises of two cuts of steak. If you would cut out the bone you would have two different steaks; a tenderloin steak and a New York Strip steak, or top loin.
The USDA has specifications for a steak to be qualified as a Porterhouse; the tenderloin steak portion has to be at least 1.25 inches thick at its widest point. A Porterhouse steak is perfect for grilling, and because of its size you will need a pretty big appetite, otherwise it is the perfect steak to share.
A really great steak like a Porterhouse needs a really good steak knife. These steak knives are my all time favorite.
image courtesy of stu_spivack/flickr