A Chateaubriand Roast is a tenderloin roast that has both ends removed, leaving the thickest, centre portion. It will weigh about 12 oz (350g) and is meant to serve two people, though it will really do about 3 people who are lighter-eaters. As it is a very small roast, it is sometimes referred to as "Chateaubriand Steak."
Occasionally you will see reports of less scrupulous supermarkets labelling Top Sirloin Steak as "Chateaubriand Steak" so that they can upgrade the price.
There are many variations on the story of how the dish actually got its name. Most are variations on it being "invented" for and named after French author François Chateaubriand, by his chef Montmireil. Some have Chateaubriand living at the time in England as a refugee from the French Revolution in a manor also called "Chateaubriand", some have him living in England as French Ambassador to the Court of St James. Others have him living in Paris at the time.
It is likely, though, that all of these stories are spurious, as unfortunately most of the easy explanations in food history are. Alan Davidson, in "The Penguin Companion to Food" (London, 2002) writes: "A tedious accretion of tales about the origin of the name was robustly hacked out of the way by Dallas (1877) in Kettner's Book of the Table..."