In a Nutshell
Although herbs and spices tend to be used interchangeably, they do have important differences. Herbs are only obtained from the leafy part of a plant while spices can come from any other part of the plant. Salt, even though widely used with food, is neither an herb nor a spice as it is an inorganic mineral.
The Whole Bushel
Humans have been using herbs and spices for a variety of reasons for thousands of years. While they are mainly used today for flavoring food, some herbs and spices boast medicinal properties and are also sometimes used for creating perfumes and essential oils. They were even once used as currency in many parts of the world. Nutmeg was once literally worth its weight in gold, and Attila the Hun requested a ransom of pepper when he besieged Rome in the fifth century.
Herbs are the leafy parts of a plant that die down after each growing season. Many plants are technically herbs, but only some of them are used for food and medicine. They can be used fresh or dried, the dried form generally lending a much stronger flavor. Herbs are typically used in larger amounts than spices, which tend to pack quite a bit more punch.
Spices, on the other hand, have a much broader spectrum of origin and can be utilized from any other part of a plant such as the roots, bark, flowers, fruit, and seeds. Spices are always used in dried form and have also traditionally been used as a preservative, as there are several spices that are known to have antimicrobial, antiseptic, or antibacterial properties. Archaeologists have found evidence of spices used for embalming in Egyptian tombs dating back to 3000 B.C. Cinnamon, cloves, and mustard are examples of spices often used in preservation, while ginger has strong anti-nausea properties.
Herbs are thought to have originated in temperate climates, but today can be found mostly anywhere. Spices, however, are native to tropical climates and most of them are still concentrated in those areas today. Spices have played an important role in the development of several countries within those areas.
Sometimes a single plant can be the source of both an herb and a spice, or more than one spice. Nutmeg and mace (both spices) for example, are both derived from the seed of the fruit of the Myristica fragrans, or nutmeg tree. The seed has a waxy red outer layer (called the “aril”) which is carefully removed, dried, and ground to make mace. The rest of the seed is then dried out and sold whole or ground to be used as nutmeg.
Coriandrum sativum, the coriander plant, is another example of a plant that produces both an herb and a spice. The leafy green part is known as coriander leaf (typically known as cilantro in the Americas), while the dried seeds are sold whole or ground as coriander.
Contrary to common belief, Pimenta dioica, or allspice, is not a blend of spices but a single spice. The unripe fruits of the Pimenta dioica are harvested, dried, and sold as allspice. It is called allspice because of its unique combination of the flavors and aromas of the spices cinnamon, nutmeg, and clove.
Show Me The Proof
Iowa State University: Herbs vs. Spices
Antimicrobial Effects of Spices and Herbs
Missouri Botanical Garden: Coriandrum sativum
Kew Royal Botanical Gardens: Pimenta dioica
Ancient History Encyclopedia: Pepper