A herb is a plant that is used for its flavor, scent, and/or medicinal qualities. As such, herbs are usually considered as edible (those used for medicinal or spiritual purposes can sometimes be poisonous when consumed in even low quantities).
Culinary herbs are distinguished from other edible plants (i.e. vegetables) by their use. Vegetables and fruit are consumed for food and nutrition. Herbs are added for flavor or scent, and add little nutritional content to the food.
Culinary herbs are a popular kind of plant for gardeners - especially those with limited space as they don't usually require much space and fresh herbs have a superior taste to dried grocery-bought herbs.
Medicinal herbs are consumed in culinary 'spicing' levels for real or alleged medicinal properties. Many of these are toxic when consumed in larger quantities (e.g. St. John's Wort and kava). Modern pharmaceutical medicine has its origins in herbal medicine (e.g. aspirin/salicylic acid from willow), but many medicinal herbs continue to be untested. Many are undoubtedly helpful, but most are probably ineffective and a few toxic examples probably remain to be identified. Therefore caution should be used when taking herbs for medicinal purposes - especially obscure herbs or if you are taking them in larger quantities.