Herbs are broadly defined as any type of plant material used for scent or flavor or medicinal or spiritual purposes. Thousands of herbs are used all over the world for these purposes. In cooking, herbs are used primarily for adding scent and flavor to foods. Most types of herbs used in cooking are leaves of some type of plant, but some herbs are flowers as well.
Parsley is perhaps the most commonly used herb and is found in dishes from all over the world. There are several varieties, including curly parsley and flat-leaf parsley. It is somewhat unusual amongst herbs, in that it is often used to garnish plates in restaurants. While this usage is not unique to parsley, the reason for it is. Parsley is high in chlorophyll and other natural compounds that help to mitigate unpleasant breath odor after a meal.
Leafy herbs are some of the most popular types of herbs. Basil and oregano are familiar to anyone who has enjoyed Italian cuisine, and sage, rosemary, and thyme are staples of French cooking. Cilantro is common in Mexican cuisine, and exotic herbs like lemon grass and specialized varieties of basil are common in Asian dishes. While thousands of different herbs are used around the world, these are some of the most common.
Many of these and other leafy herbs also bear flowers that can be used to provide scent or flavor to food. Lavender is one example of this. Its flowers have been used for centuries to make flavorings for candies and pastries as well as in the manufacture of perfumes, potpourri, and scent packets. Nasturtium flowers have a peppery flavor, and fresh blossoms are often used to flavor salads.
Herbs are often available in dried form. These herbs will impart a flavor to to foods, though it is nearly always inferior to the flavor from fresh herbs. Many chefs and home cooks prefer to use fresh herbs whenever possible but will resort to dry herbs if fresh are not available. Fresh herbs contain more of the essential oils that give the herb its flavor and character. These oils tend to degrade over time, and the herbs lose their potency when dried and as they age.
Other types of herbs are widely used for spiritual and medicinal purposes. Nearly every culture has a store of knowledge about herbs and their uses that has been passed down for generations. Many of these properties are well documented by modern science and are not just anecdotal. Some modern medicines and treatments are derived from herbal sources. Digitalis, for example, is widely used to treat heart conditions and is extracted from the foxglove plant, a popular garden flower.