Fennel seeds are the seeds of the fennel plant (Foeniculum vulgare), a yellow-flowering species in the same family as carrot, anise, parsnip, and coriander. Fennel is native to the Mediterranean, and has grown there for millenia. The ancient Greeks and Romans used fennel, and it figured into their mythology as well.
Fennel seeds resemble aniseed, but are larger, and have more of a green coloration. They are used as a breath freshener, and are important in the cuisines of India, the entire Mediterranean basin, Lebanon, and China (where is it a part of five spice powder).
Fennel seeds are hard, and should be muddled before being utilized in cocktails. They have a warm green hay-like aroma, subtle licorice flavors, and a slightly bitter, spicy finish.