Wool is the classic yarn fiber. It is springy and warm, easy on the hands, has good stitch definition, and stays warm when wet. Sub-types of wool include Merino, Bluefaced Leicester (BFL), Polwarth, Rambouillet, and Targhee.
Superwash or not? Superwash yarn is more washable and offers more vibrant colors, but not everyone is in love with the chemical-intensive process that produces it. Also, projects made with superwash yarn can grow when washed. Non-superwash wool (a.k.a. wool) can be felted (whether on purpose or not...).