Herbs are derived from plant parts in order to make medicines, seasonings for food, teas, perfume sachets, etc.
In cooking, the leaves of plants are used fresh or dried to help improve the flavor of a recipe. Basil, oregano, bay and parsley leaves are needed to make traditional Italian red sauce and provide a great deal of flavor. Rosemary leaves are used as seasoning in various meat recipes to bring out flavor.
Many drugs are derived from different parts of plants and these are herbs used for medicinal purposes. These include many of the remedies seen in health food stores like St. John's wort--an herbal supplement that affects mood, saw palmetto which helps people who suffer from enlarged prostate and green tea which is known to lower cholesterol. Garlic is known for improving cholesterol levels.
Lavender can be used to make perfumes and sachets. Herbs like mint are used as flavorings and tea. Some herbs like coca and cannabis have been chewed and smoked for centuries to relieve pain. In aromatherapy, combinations of various herbs have soothing effects for stress, or can be used to make someone feel more awake when lethargy is a problem.
Herbs have been studied for their effects on various diseases including cancer. Many drugs that are used in traditional medicine are derived from plants with new ones being researched all the time.