This page is taken directly from the FBI guide to the National Instant Criminal Background Check System

http://www.fbi.gov/about-us/cjis/nics/general-information/nics-index

In November 1993, the Brady Handgun Violence Prevention Act of 1993 (Brady Act), Public Law 103-159, was signed into law requiring Federal Firearms Licensees (FFL) to request background checks on prospective firearm transferees. The permanent provisions of the Brady Act, effective November 30, 1998, required the U.S. Attorney General to establish the National Instant Criminal Background Check System (NICS). FFLs may contact the NICS by telephone, or other electronic means, to determine whether or not the transfer of a firearm would violate Section 922 (g) or (n) of Title 18, United States Code (U.S.C.), or state law.

The NICS is a national system that checks available records in the National Crime Information Center (NCIC), the Interstate Identification Index (III), and the NICS Index to determine if prospective transferees are disqualified from receiving firearms.

Prohibiting Categories Defined by Statute

The NICS Index contains information that may not be available in the NCIC or the III of persons prohibited from receiving firearms under federal or state law. A valid match of a NICS Index record to a prospective firearm transferee results in an immediate determination of firearm dis-qualification. NICS Index records are voluntarily provided by local, state, tribal, and federal agencies.

Section 922(g) of the Gun Control Act prohibits certain persons from shipping or transporting any firearm in interstate or foreign commerce, or receiving any firearm which has been shipped or transported in interstate or foreign commerce, or possessing any firearm in or affecting commerce. These prohibitions apply to any person who:

18, U.S.C. §922 (g) (1)
Has been convicted in any court of a crime punishable by imprisonment for a term exceeding one year

18, U.S.C. §922 (g) (2)
Is a fugitive from justice

18, U.S.C. §922 (g) (3)
Is an unlawful user of or addicted to any controlled substance

18, U.S.C. §922 (g) (4)
Has been adjudicated as a mental defective or committed to a mental institution

18, U.S.C. §922 (g) (5)
Is an alien illegally or unlaw-fully in the United States or who has been admitted to the United States under a nonimmigrant visa.

18, U.S.C. §922 (g) (6)
Has been discharged from the Armed Forces under dishonorable conditions

18, U.S.C. §922 (g) (7)
Having been a citizen of the United States, has renounced U.S. citizenship

18, U.S.C. §922 (g) (8)
Is subject to a court order that restrains the person from harassing, stalking, or threatening an intimate partner or child of such intimate partner

18, U.S.C. §922 (g) (9)
Has been convicted in any court of a misdemeanor crime of domestic violence

18, U.S.C. §922 (n)
Is under indictment for a crime punishable by imprisonment for a term exceeding one year

Please reference the Federal Register, Volume 62, Number 124, Rules and Regulations, for more complete definitions of the prohibiting categories.

Submission Process

The local, state, tribal, and federal agencies are provided with two modes to add, modify, supplement, or cancel NICS Index entries.

The first mode is an electronic connection between the NICS and the contributing agency using the NCIC-Front End. An agency may use this interface to electronically submit, modify, supplement, cancel, or display a denied person’s disqualifying information in the NICS Index. The second mode is batch data transfer on CD, diskette, or by secure e-mail. The NICS Index entries, modifications, supplements, and cancellations are to be processed by the contributing agency since the contributing agency is responsible for the accuracy and validity of the NICS Index information. It is imperative to update the NICS Index as necessary to minimize erroneous denials. The contributing agency is responsible for responding to appeals, assessments, and audits of the submitted records.

In addition to local, state, tribal, and federal agencies voluntarily contributing information to the NICS Index, the NICS Section receives telephone calls from mental health institutions, psychiatrists, police departments, and family members requesting placement of individuals into the NICS Index. Frequently, these are emergency situations and require immediate attention. Any documentation justifying a valid entry into the NICS Index must be available to the originating agencies.