You can use the service for a variety of different purposes. For
example, Y!Q uses it to
determine key concepts within the search context and then uses those
terms for augmenting a user's search query.
There are more cool things you can do with this, and we're hoping
to see many interesting applications that use the term extraction
service. For example, a publisher might use the service to extract key
concepts from an article (e.g., a blog post). The publisher could then
use this information to highlight those terms in the text and make
them clickable, or even insert Y!Q at those points.
Let us know what you end up building with this service...