It is available in many variations but the basic is thinly sliced beef, slightly simmered with onions in the omnipresent dashi (出汁) stock and laid over a big portion of rice. It is served in a big bowl called “donburi” (丼) or, more simply “don” hence the name “gyudon” (牛丼) i.e. “beef bowl” and accompanied by a cup of miso-siru (味噌汁) i.e. “soup flavored with miso/fermented soy” is probably the simplest and cheapest meal you can get anywhere in Japan.
(For a bigger version of this picture both in color and black and white, check my "Japan Arekore" set on Flickr)