The content below is from a great post at Learning SharePoint

Here is a little comparison chart for describing Search capabilities in Search Foundation 2010 Vs  SharePoint Server 2010 Vs FAST Search Server 2010

Feature	SharePoint Foundation 2010	SharePoint Server 2010	FAST Search Server 2010
Basic Search	Yes	Yes	Yes
Document preview	N	No	Yes
Indexing sites	Yes	Yes	Yes
Indexing external content	No	Yes	Yes
People search	No	Yes	Yes
Query federation	No	Yes	Yes
UI-based administration	Limited	Yes	Yes
Visual best bets	No	Limited	Yes