Katsudon is a type of donburi, meaning it's a rice dish served in a bowl topped with simmered ingredients. In this case, the topping is another Japanese favorite — tonkatsu — a deep-fried and crumbed pork cutlet, but here it is simmered together with vegetables in a seasoned sauce bound with eggs.
The earliest reference to the dish dates back to 1921 and since then numerous varieties have been developed by adding or replacing the ingredients from the original recipe. The best-known versions include katsudon made with Worcestershire sauce, miso, and soy sauce, but there are also ones that replace pork with beef or chicken.