Japanese curry rice is quite different from Thai or Indian curries. The basic ingredients are the same: a plate of rice with curry sauce ladled on top. But Japanese curry is thicker and creamier -- it's more of a rich sauce that's been thickened with a roux.
And while you generally have to strap yourself in and get ready for a very spicy ride with other types of curries, Japanese curry is, by default, sweeter than it is spicy -- although any good curry joint will let you crank up the spiciness.
When we think of Japanese foods, we think sushi, teriyaki, tempura. We don't think about curry, which is amazing when you consider just how popular it is in Japan. There are several lunch counter style chains that specialize in nothing but curry rice, serving it up lightning-fast for businessmen on tight lunch breaks. Moms make it at home regularly.
You also don't have true curry rice unless you have toppings. The traditional topping is a tonkatsu, a breaded, fried pork cutlet. Other popular toppings include chicken katsu, sausage, boiled eggs, fried shrimp, and cheese.