The T-bone steak and porterhouse steak are prime steaks of beef cut from the short loin. Both steaks include a "T-shaped" bone with meat on each side. Porterhouse steaks are cut from the rear end of the short loin and thus include more tenderloin steak, along with (on the other side of the bone) a large strip steak. T-bone steaks are cut closer to the front, and contain a smaller section of tenderloin.
Owing to their large size and the fact that they contain meat from two of the most prized cuts of beef (the short loin and the tenderloin), T-bone steaks are generally considered one of the highest quality steaks, and prices at steakhouses are accordingly high. Porterhouse steaks are even more highly valued owing to their larger tenderloin.
In British usage, followed in Commonwealth countries, porterhouse refers to the strip steak side of a T-bone steak, while the tenderloin side is called the fillet.