The Korean chili powder used as a key ingredient in kimchi and gochujang is gochugaru.
Gochugaru is made from dried Korean red chili peppers. It is sold as either a fine powder or coarser flakes, and its flavor is typically described as spicy, mildly sweet, fruity, and slightly smoky. Heat levels vary considerably, so the same amount can produce very different results depending on the peppers used.
The word comes from Korean: gochu means chili pepper and garu means powder. Gochugaru is essential to many kimchi recipes, where it contributes color, heat, and pepper flavor. It is also one of the principal ingredients in gochujang, the fermented Korean red chili paste, along with glutinous rice, fermented soybean powder, malt, and salt.
Gochugaru is not the same thing as gochujang. Gochugaru is a dry pepper seasoning; gochujang is a thick fermented paste that contains gochugaru among several ingredients. Ganjang and doenjang are also different: they are soy-based sauces or pastes rather than chili powders.