“Well, without going through a litany of texts, let’s just say that it appears as though most instructional authors have bought into the precept that “everyone” already knows enough about peacock herl and/or that all one needs to know is that it is very popular…”

“The distinction between feather, quill, and herl is important when tying flies. It is also important when buying the material. If you walk into a shop and say you want to buy peacock, you will need to specify what you want or they will usually assume you want “herl” and you’ll end up with a package of strung herl; which gives you the quills once you strip the herl. You will need to say you want peacock eye or peacock herl. With the eye, you get some herl; but, with a package of herl, you do not get the eye. Then, there’s peacock sword. But, that’s a discussion of a different feather…”   HERL

Good overview of peacock as a feather, herl and quill. The site has other good informational pieces for beginning fly tiers.