Beef Hearts are very large, so they are rarely sold whole. The photo example, from an Asian market, appears to be about 1/3 of a whole heart, and with most of the plumbing cut off. It weighed 1 pound 6 ounces, and was 7-1/2 inches long, 4-3/4 inches wide and 1-3/4 inches thick. Since most Americans don't want to buy any meat that actually looks like part of an animal, most regular markets sell heart sliced about 1/2 inch thick with most of the fat cut off.
This is muscle meat, and very well exercised, even tougher than shank - simmering time will need to be about 3 hours. Hearts are made of a unique type of muscle, called cardiac muscle. It is a striated muscle like skeletal muscle, but this is not obvious, it looks smooth.
More on Beef Innards
Buying: Most Beef Hearts are ground up and included in various sausages and similar products, but some do appear in better quality meat markets. The price is reasonable because most people don't know what to do with them. The photo specimen was purchased from a large Asian market in Los Angeles for 2016 US $1.99 / pound.
Prep: Generally all that is needed is to cut off the thick fat at the top and any remaining plumbing on the inside - then slice as desired.
Cooking: Beef Heart needs to be wet cooked for a long time, about 3 hours at a simmer, so is used mostly in stews and similar dishes.
Yield: With the thick fat cut off, the heart in the photo above yielded 91%. Heart meat purchased sliced may yield about 95%, depending on how picky you are.