Herbs are any plant used for flavoring food, medicine or perfume and in some cases herbs are used in a spiritual sense. Some herbal plants have flowers and some just have foliage. Depending on the particular herb any part of the plant maybe used from the leaves to the flowers to the root.
Culinary herbs are different from vegetables in that, like spices, they are used in small amounts that add flavor rather than substance to food. As an example, cilantro can be added to salads to give it more punch or in the use of pesto, basil can be used in moderate amounts to add the primary flavor of pesto. Culinary use typically distinguishes herbs as referring to the leafy green parts of a plant (either fresh or dried), but it is not uncommon to use the flower of an herb in some recipes, however it is uncommon to use the root.
Spiritual uses of herbs again are use in a different manner and are often used in the form of burning to release the aroma into the surrounding area . The essential oil of the herb is also commonly used in religious ceremonies for anointing.
Medicinal Herbs have been used since the beginning of time. Plants contain phyto-chemicals that have an effect on the body and based on certain properties of the plant it can be used for a variety of maladies. One such example is the herb peppermint which is often used for digestive upsets and even headaches.
As with all things it is good practice to learn about the plants that you use to ensure safety.