Herbs and spices are interchangeable a lot of the time, with many herbs or spices being known as one or the other. There is no strict standard to deciding what an herb is and what a spice is. Consensus says that an herb is either the fresh or dried leaves of a plant that does not have a woody stem, though it is not always the case. Spices, on the other hand, come from a range of sources including the roots, bark, berries, seeds and stems which are then dried.
Herbs have the additional use of being used for medicinal purposes, while spices do not. Both however are used for religious purposes, such as in offerings, a prime example being the burning of incense which can be made of both herbs and spices.
Common spices, such as curry, are actually a combination of spices. Herbs do come as combinations, but those are sold purely as a marketing ploy. Italian Seasoning, for example, is nothing more than a combination of oregano, basil, marjoram, thyme, rosemary and sage. The combination differs among brands.
If you are still having difficulty differentiating between herbs and spices, the easiest thing to do is check your fresh produce section at your local grocery store. There, you will be able to find entire herb plants for sale, which you can keep in their own containers or plant in your garden. Alternatively, the store will offer fresh-picked herbs in small plastic containers. Either way, the herbs will be fresh and great to use in any meal. All the spices you'll find in the spice aisle.
Exceptions to the Rule
There is always an exception to the rule, which is why there is no hard and fast definition of what is an herb and what is a spice. Sage, for example comes from a plant that has a woody stem and is most commonly used dried.
Thyme is another example, as it resembles pine needles, so one could argue it comes from a plant with a woody stem, but is used as an herb.
Salt: Not a Spice
Salt is not a spice, but it is found in the spice section. The reasoning behind this is the fact that it is commonly used as a spice rather than a preservative. In times past it was used as a preservative. In today's world, salt is used for the flavor it provides but based on the definition of a spice given above salt does not qualify. Salt is in fact a mineral that is either mined or derived from saltwater. For more information read the book "Salt: A World History" by Mark Kurlansky.
Herbs and spices come in many varieties. Pepper and mint are two prime examples where there is a basic class name for each, but within the class are a selection of flavors. Do not be fooled into thinking that they are interchangeable, as the flavors each invokes can be quite different and readily change your meal to something you didn't intend.
If you are out of one type of pepper or mint, consult a cookbook or other source first before replacing it with its cousin. For an excellent visual tool, the FoodPairing website shows you the connection between various foods, fruits, herbs and spices and how they can all be replaced (see Resources).
The same goes for dried berries and other similar spices. Elderberries would be no substitute for juniper berries and vice versa.
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