In botany, an herb is a non-woody plant that flowers and sets seed and then dies back to the ground as opposed to a tree, a shrub, a vine, etc.

For everyone who isn't a botanist, the word herb refers to any plant that is useful to humans. This includes plants used for flavoring food, plants used for medicine, plants used for aromatherapy, plants from which fibers, dyes and other crafting materials can be gathered and plants that can be used for magickal purpose.