Castoreum is the flavoring from the beaver anal glands. You'll never want to eat anything with natural vanilla (or some fruit) flavored items. It is listed as natural flavoring.
http://en.wikipedia.org/wiki/Castoreum
Castoreum is used as an ingredient when making flavoring.
If a label says 'Flavored' that means its a fake. Manufactured to sort of resemble the real thing. If the label says vanilla or natural vanilla it comes from real vanilla beans.
They can list castoreum as a natural ingredient in vanilla flavoring. But can't list vanilla flavoring as a natural ingredient in something like ice cream.