Gyudon, stewed thin sliced beef on rice is probably most popular Japanese fast food. Gyudon is a type of donburi-mono, a meal which comes on rice. Donburi actually means a bowl and mono means stuff/thing.
Gyudon is a fast food due to it is not so much cooked at home. Also it is not available at ordinary restaurants. Usually people eat at diners specialized in gyudon which is nationwide chains. Sukiya, Yoshinoya and Matsuya are the biggest three chains in Japan. These diners offer wide varieties of gyudon with numerous types of flavors. Then, you can also add small dish or miso soup to them.
Also you have to choose the size of the gyudon. Usually the sizes are nami - regular, oomori - large, tokumori - XL. Oomori has bigger portion of rice but same amount of beef as regular. Tokumori has both bigger portion of rice and beef.