Introduction to Federated Search by Jill Hurst-Wahl, FUMSI (Oct 2008)
Federated search, as this article so clearly describes, is "a discovery tool that allows the user to find information quickly from multiple sources". On the Web we know it as metasearch, and Vivisimo / Clusty has been an example of this for many years.
Federated search software is bought by organizations who are looking for a single interface to search across many data sources in the organization. Jill Hurst-Wahl gives us a very clear and concise account of what the software does, the features it may have, shortcomings and benefits, and considerations for choosing.