Mustard is a spice made from the seeds of Brassica juncea and related species. It is yellow or yellow-ocher in color and has a pungent taste. It's generally categorized as Japanese mustard or plain mustard (Western). There are also white and black mustards among the Western mustards. In Japan, "karashi" refers to Japanese mustard and the Western-type mustard is called simply "mustard."
Mustards are easier to handle, but there is a clear distinction between the flavor of Japanese and Western mustards, so when one is mistakenly used the effect may or may not turn out as planned. Japanese mustard has a distinctive bitterness and an astringent aspect that needs to be removed before use, which is another point of distinction from mustard. Taking advantage of this difference in flavor, Japanese mustard is specifically used for sandwiches and hamburgers in Japan.
The name "Japanese mustard" may imply that it originated in Japan, but in reality mustard is a spice from central Asia that was brought into Japan via India and China.