Hop (Humulus lupulus L.) is a species of Humulus in the Cannabaceae family. It is a perennial industrial plant originating in Asia. The cones contain lupulin glands, which are responsible for the secretion of lupulin, an aromatic, bitter and sticky  yellow resinous powder found on the loose cone scales.
For industrial purposes, only female plants are grown. The hop plant itself has four major components: The crown which consists of the roots and rhizomes (the rhizome is an underground stem that stays in the ground from year to year and is the real perennial part of the plant); the central stem known as the bine or vine; the leaves; and the flowers or hop cones.
Hops are harvested in late summer (from mid-August to early September) when the level of industrially relevant substances in the hops is highest.
Today, hops are used as a vital commercial ingredient in the beer industry and, to a lesser extent, in herbal medicine.
It is the hops that give beer its pleasantly bitter flavour and aroma.
(Source: Assoc. Prof. Barbara Čeh Brežnik)