Coastal: These are the regions that lie near the sea, oceans, or possibly near large inland salt-water lakes. As a rule of thumb, I assume that the coastal regions can extend up to five miles inland, although this may vary from place to place.
Desert: Most people assume deserts to be very hot places, such as the Sahara, the Australian Desert, or Death Valley, and indeed many deserts are very hot, but there are also cool deserts. A desert is normally defined as any area that receives, on average, less than 10 inches of rain a year, but for game purposes may be assumed to be any area that is very dry, without recourse to such official formulae.
Forest: A forest is any area that is heavily covered with trees. There are both large forests, which may cover vast areas, or small forests. In medieval times, forests covered a much larger area of the world than they do today.
Grassland: These areas, while mostly untouched by agriculture contain very few large tress. Instead they are mostly covered with grass, hence their name. The American prairies are a good example of grasslands.
Hills: A hill is a relatively small raised area of land. They are often found at the foot of mountains, but may exist in their own right, separate from the surrounding landscape.
Jungle: Similar to a forest, but normally thicker in nature, and found in more tropical environments. The Amazon is a perfect example of a jungle environment.
Mountains: Very large, raised areas of land, often found at the borders of continental shelves. A notable feature of mountains is that they are, due to their height, often much colder than the surrounding land. Another feature is the rain shadow phenomena. It may rain very heavily on one side of a mountain, but not at all on the other. The Himalayas, the Alps, and the Rockies are all examples of mountain chains.
Rivers: A river is a flowing stream of (normally) fresh water which runs to the sea. Rivers can be large (like the Mississippi, or Amazon) or very small, local creeks.
Rural: This describes farmland, land that has been turned over to agriculture, but is only sparsely settled.
Special: Where this is listed as the locale, read the description of the herb for more details.
Swamp: Areas of very moist soil. The Florida Everglades are a good example of a swamp.
Underworld: By Underworld, I refer to large underground complexes, ie the Underdark of Advanced Dungeons & Dragons, the Mac Mordain Cadal from the works of Raymond E Feist, Ulgo from the works of David Eddings, or the Mines of Moria from Tolkien. It might also be assumed that the herbs may grow in smaller complexes, but I do not recommend it.
Urban: Any area that is widely settled, such as towns, villages or cities.
Volcanoes: When a herb is said to grow in this locale, it may grow actually within the mouth of the volcano, or merely on its sides (GMs discretion, unless the description makes it clear).
Wasteland: This is the hardest area to describe. A wasteland may have once been any type of land, but has now been rendered totally inhospitable perhaps by some great disaster. Much of France after World War I fulfilled this criteria, as did Tunguska after the 1908 explosion. Hiroshima and Nagasaki, or Chernobyl also fulfilled this criteria. In essence, any area of your gameworld where some great disaster rendered the land uninhabitable, may fit into this locale.