I see someone already said the hair and feather part, but thee is even more to it than that as there are several different kinds of hackle and even synthetic hackle. Just sticking to feathers though there is neck hackle which is what most use on jigs for a tail then there is saddle hackle mostly used for a collar or palmering over the body. Also then there is hackle used for dry flies and it's used by size meaning across the feather and even measured by hook size and these are really thin feathers where neck hackle is wide and webbey.
So there are a lot of different hackle feathers and then the synthetic type as well.