Korean chili flakes
Coarsely ground, sun-dried red chilies without seeds, bright red, fruity and only moderately hot.
Korean chili flakes, called gochugaru in Korean, are ground from dried red chilies, usually with the seeds and membranes removed. They are coarser than cayenne and noticeably milder than ordinary chili flakes, with a fruity, slightly sweet and faintly smoky note. Because they burn little, they are used by the spoonful and colour food a deep red, which is exactly what makes them indispensable for kimchi, stews and marinades. There is a coarse grind for kimchi and a fine powder for sauces and pastes. They are sold in Asian grocery shops and by mail order, usually in larger bags, which should be kept cool, dark and airtight or in the freezer once opened.
Substitute:
There is no truly equivalent substitute. The closest are Turkish chili flakes (pul biber) from a Turkish grocery, somewhat hotter and saltier, or sweet paprika with a small pinch of cayenne, which matches the colour and heat but not the fruity, smoky depth. Ordinary chili flakes are far hotter and must not be used in the same quantity; for kimchi the real thing is worth seeking out.