FAST Search Server for SharePoint is an optional search solution for SharePoint 2010.
FAST Search Server for SharePoint, often abbreviated FS4SP, combines the power of FAST with SharePoint in order to create more powerful search based solutions.
FS4SP introduces advanced content processing, deep refiners, contextual search, multiple relevance profiles and a powerful query language which supersedes the capabilities of the out of the box search capabilities in SharePoint 2010.
A feature comparison is listed at the Microsoft SharePoint page.
