An
accumulation of dead
skin cells,
skin oils and probably just plain 'ol
dirt that forms on your
scalp if you don't
wash it regularly. Can also happen in you
don't shampoo your hair.
Ever since I read the don't shampoo your hair writeup, I've been living in disgusted fear of cradle cap. Mind you, my hair is now less than an inch long all around, and I shampoo daily, being extra careful to scrub the scalp diligently. In fact, when I got my hair cut, I could feel the sebum from my scalp washing out the first time I washed it after that, because my hair was longer (3-4 inches) and I couldn't get at the scalp as well as I would have like. It wasn't a full grown cradle cap, but still, it grossed me out.
Now, to each their own, and I know people say with dreadlocks who probably have cradle cap as well, but I could never be one of them. Eeew. This is one aspect of personal hygiene that I'm a firm believer in modern technology and social practices.