Types of vanilla beans
There are two species of vanilla beans - Planifolia and Tahitensis. Planifolia is the flavor that people are most familiar with - this is what supermarket vanilla extract is made from. Tahitensis is a mutation from Planifolia and adds a sweet floral note.
Most Planifolia comes from Madagascar, although it is grown around the world in Mexico, Uganda, India, and Indonesia to name a few. Planifolia is commonly referred to as Madagascar Bourbon or Bourbon beans (Bourbon was an island off of Madagascar, now called Reunion).
Tahitensis, commony referred to as Tahitian vanilla beans, originated in (of course) Tahiti. Due to its size, Tahiti does not produce much vanilla beans, and hence they are quite expensive - about 6 times as much as Tahitian beans grown in other locations. Papua New Guinea is a large producer of Tahitian vanilla beans.
There are two grades that most people are familiar with: Grade A Prime Gourmet, which is about 30% moisture, and Grade B Extract, which is about 20% moisture. Grade A should be used for cooking and can be used for making extract. Grade B should be used for making vanilla extract. Grade B is usually too dry to scrape the caviar out, although you might be able to do so with some beans.
Where a beans is grown and how it is cured affects the taste.