The National Information Exchange Model (NIEM) is an XML-based information exchange framework for sharing data between communities of interest (COIs), across all levels of the United States government. COIs may include law enforcement, the court system, public safety, intelligence, homeland security and emergency and disaster management.
Instead of trying to integrate local, state, tribal and federal databases, NIEM standardizes a semantically consistent information exchange package by providing a common format and language for describing people, places, things & events, as well as the relationships between them.
NIEM is a partnership between the U.S. Department of Justice and the U.S. Department of Homeland Security. It is built on an exchange framework developed earlier for the criminal justice community called the Global Justice XML Data Model.
The NIEM.gov website has instructions for implementing NIEM.
Marc Clifton and Mark Long provide an excellent Introduction to NIEM and IEPDs.
Wikipedia provides a more technical overview of the National Information Exchange Model.