DEFINITION:
In general use, herbs are any plants used for food, flavouring, medicine, or fragrances for their savoury or aromatic properties. Culinary use typically distinguishes herbs from spices. Herbs refers to the leafy green or flowering parts of a plant (either fresh or dried), while spices are produced from other parts of the plant (usually dried), including seeds, berries, bark, roots and fruits.

In botanical English, the word “herb” is also used as a synonym of “herbaceous plant”.
Herbs have a variety of uses including culinary, medicinal, and in some cases, spiritual. General usage of the term “herb” differs between culinary herbs and medicinal herbs. In medicinal or spiritual use any of the parts of the plant might be considered “herbs”, including leaves, roots, flowers, seeds, root bark, inner bark (and cambium), resin and pericarp.

Culinary herbs are distinguished from vegetables in that, like spices, they are used in small amounts and provide flavour rather than substance to food.
Herbs can be perennials such as thyme or lavender, biennials such as parsley, or annuals like basil. Perennial herbs can be shrubs such as rosemary, Rosmarinus officinalis, or trees such as bay laurel, Laurus nobilis – this contrasts with botanical herbs, which by definition cannot be woody plants. Some plants are used as both herbs and spices, such as dill weed and dill seed or coriander leaves and seeds. Also, there are some herbs such as those in the mint family that are used for both culinary and medicinal purposes.

Some plants contain phytochemicals that have effects on the body. There may be some effects when consumed in the small levels that typify culinary “spicing”, and some herbs are toxic in larger quantities. For instance, some types of herbal extract, such as the extract of St. John’s-wort (Hypericum perforatum) or of kava (Piper methysticum) can be used for medical purposes to relieve depression and stress. However, large amounts of these herbs may lead to toxic overload that may involve complications, some of a serious nature, and should be used with caution. Source Wikipedia