It augments both the indexing AND the querying aspects of Search. It’s an add-on, sitting on top of SP native search, and it provides extremely rich search functionality. I like to suggest that folks look at the ‘shopping’ experience at http://www.bestbuy.com. That was built with FAST… it’ll help you visualize what it can do.
There are two products – FAST Search (standalone) and FAST Search for SharePoint…. FAST Search for SharePoint integrates into your SharePoint environment to provide enhanced search indexing and query time user experience.
<i>This question was answered by Spencer Harbar, Enterprise Architect at Microsoft and Dan Holme, Consultant and Trainer, Intellium.</i>