Gochugaru
Korean chili flakes made from sun-dried red chilies, ground coarse or fine. Bright red, fruity, mild to medium hot and slightly smoky.
Gochugaru is dried, seeded red chili ground into flakes or a fine powder. It is the essential form of chili in Korean cooking, giving kimchi, stews and marinades their colour and heat. The flavour is fruity and faintly sweet with a light smoky note, and the heat stays moderate, so it can be used in large amounts without making a dish fiery. The coarse grade suits kimchi and seasoning pastes, the fine grade sauces and even colouring. It is sold in Asian grocery stores and online; kept in the fridge or freezer it holds its colour and aroma much longer.
Substitute:
The closest match is Aleppo pepper (pul biber) from a Turkish grocery, which is similarly fruity and mild but oilier, often salted and slightly tangier. In a pinch, mix sweet paprika with a little cayenne; this roughly matches colour and heat but not the fruitiness or texture. Ordinary crushed red pepper flakes are too hot to use in the usual quantities.