What is the difference between an herb and a spice?
The terms "spice" and "herb" have both been used to describe parts of plants (possibly dried) that are used to enhance the flavor or taste of food. In addition, herbs have been used to augment cosmetics, preserve foods and cure illnesses.
Spices and herbs can consist of flower buds, bark, seeds, leaves or many other parts of a plant. Over time the definitions for spices and herbs have changed a bit. In the past, spices have been categorized as fragrant, aromatic plant products like cinnamon, cloves, ginger and pepper. These spices are found in plants grown in tropical and subtropical regions of the world. While herbs have always been recognized as the more green, leafy products like mint, rosemary and thyme grown in more temperate areas.
But according to the American Spice Trade Association, today spices have become known as "any dried plant product used primarily for seasoning purposes." This all-inclusive definition seems to cover a wide range of plants like herbs, spice seeds and even dehydrated vegetables and spice blends.
—The Fact Monster