How does Korean chili powder differ from "US" chili powder? I've been looking at a number of kim chi recipes and they all call for 'Korean chili powder'.  My visit to the local Asian grocer only yielded 'Asian' chili powder and other nondescript chili powders.

Is there a particular chili powder that is Korean?  Can I simply use cayenne pepper or crushed red pepper flakes?
Korean chilli is a little different as it has a slight smoky flavour, in addition to being slightly sweet and also quite hot. 

The actual name of the chilli use in kimchi and for that matter, most Korean dishes is gochugaru (고추가루). It comes in a variety of preparations, typically, finely ground, flakes and a paste. 

You should be able to find this in most good supermarkets or an Asian store. If you can't get this, you can still use a good quality chilli powder or possibly, a paste.