Kobe beef is beef made from cows that are fed a diet enriched with beer and massaged lovingly by attendants in Kobe, Hyogo, Japan. Japanese Kobe Beef is well marbled and very tender. The attention and time given to the cows is said to produce the best beef and gives Kobe beef a high cost.
There are mainly four breeds in wa-gyu (wa means Japanese and gyu means cattle): Japanese Black, Japanese Brown, Japanese Polled, and Japanese Shorthorn. Japanese Kobe Beef is from Tajima-ushi, which is one of Japanese Black. Tajima-ushi are fed on natural feed such as barley, wheat bran, and grass, and good water in Tajima region. Among Tajima-ushi, beef which meet special criteria become Kobe Beef. The beef must be ranked in Grade A or B with fat BMS number 6 and over by Japanese Association of Meat Ranking. It's said that only 3000-4000 Kobe Beef are available per year in Japan.
http://www.calsky.com/lexikon/en/txt/k/ko/kobe_beef.php
Wagyu > Tajima > Tajima-ushi > quality criterion to become "Kobe"