The National Archives (U.S.) has created a power point presentation on using the Archival Research Catalog (ARC) for genealogical research. The ARC is the online catalog of NARA's nationwide holdings in the Washington, DC area, Regional Archives and Presidential Libraries.
View here: http://www.archives.gov/research/arc/topics/genealogy/arc-demo.pdf
They also offer a second presentation with case studies showing online research on two people using ARC and AAD. The Access to Archival Databases (AAD) provides access to over 85 million historic electronic records created by more than 30 agencies of the U.S. federal government and from collections of donated historical materials. This collection is continually growing.
View here: http://www.archives.gov/research/genealogy/start-research/arc-aad-case-studies.pdf
Below are the direct links to the databases discussed in the above power point presentations:
Access to Archival Databases (AAD)
This also has a Getting Started Guide for using the AAD
Archival Research Catalog (ARC)