Although historically the name yakiniku has meant different things that could all fall under the category of barbecue, what we now call yakiniku is this Korean-style barbecue, and it emerged in the aftermath of World War II when Koreans in Japan began opening these restaurants. They were (obviously) popular, and the style spread up and down the country.
Yakiniku can include many different ingredients, but its defining characteristic is bite-sized meat and vegetables cooked at the table over a gas or charcoal grill. Beef is the classic, including many different thinly-sliced cuts.
The ingredients in yakiniku are generally pre-marinated before being brought to the table for diners to cook; different marinades exist, but the most common one is a sweet-soy tare sauce.
In beef yakiniku, the combination of marinade, meat, and grilling technique results in an absurdly delicious bite of rich umami, subtle sweetness, and satisfying sear all at once. It’s generally not cheap so isn’t exactly an everyday dinner, but it is an absolute treat. Find Yakiniku cuts of beef at Uwajimaya, available in the meat department.