Herbalists carefully harvest the helpful and potent herbs found throughout the world. As they travel, herbalists use their keen senses to seek out the rarest and most precious flora, which can be transformed by other professions' mystic recipes. Herbalists can supply themselves (or their guilds or groups) with an abundant quantity of reagents to aid in their crafting, or they can sell or trade bundles of herbs for a profit.
Herbalists can locate herbs on their map via a keen sense for the growth and placement of flora, and they can harvest them even more easily (by simply right-clicking on an herb). No special tools are required to gather herbs. As you gather a greater number of increasingly obscure herbs, your Herbalism skill will grow. High skill values are required to harvest the choicest herbs.
Herbs grow nearly everywhere in Azeroth, though some areas are more fecund than others. Certain rare herbs appear infrequently, are difficult to find, or both - these will often command high prices and power potent crafting.
Activate your 'find herbs' ability while wandering the world to identify herbs. They'll show up on your mini-map when you get near them.
When you find an herb, walk up to it and right-click on the herb to gather it. You'll receive a small cache as loot!