Herbalism is an great skill which allows you to make your own potions, poisons and drinks by combining the correct ingredients together. Each potion has a different effect when you drink, such as raising a skill or cure poison. Some poisons allow you to poison a weapon.
Besides, Herbalism is a skill for members only and you need to complete the quest Druidic ritual before you can start making potions and cleansing herbs. All potions are made with a specific grass, and another ingredient that is added to your unfinished potion and buy runescape gold.
These ingredients can be gotten by killing monsters, but most minor ingredients can also be found on the ground, things like the spider eggs and white berries that can be obtained in the desert or deep red spiders. Almost all creatures or humans in RuneScape drop herbs, but some creatures are better than others to kill if you try to get herbs.