Herbalism is one of WoW's gathering professions - its only purpose is to collect materials (mostly Herbs, some elemental reagents) required by other professions ( Alchemy and Inscription). Herbs can be found in every zone of Azeroth and Outland, with the zone's level usually corresponding to certain herbs. If you pick this profession you will find that some herbs require very specific geological or climate conditions, and this knowledge will help you gather them faster (for example, Earthroot grows on hills/mountains and Liferoot is always found near river- and lake-water).
Profession Perk
Herbalists get a relatively weak perk - like all practitioners of gathering professions. They learn Lifeblood - simple instant healing spell with a temporary haste rating buff that grows in strength every 75 skill points. An additional cosmetic perk herbalists get is the Lifegiving Seed which they have a chance to loot off every Cataclysm herb. The Seed can temporarily transform the herbalist into a random herb in an attempt to avoid notice from enemy players.