From the wiki:
The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) is a Federal law enforcement agency within the United States Department of Justice. Its responsibilities include enforcing certain Federal laws, administering certain Federal crime prevention programs, protecting the public and reducing violent crime.
The ATF enforces Federal laws and regulations relating to alcohol, tobacco products, firearms, explosives, and arson.
Two incidents in the early 1990s brought criticism to the agency, as well as to the Federal Bureau of Investigation (FBI) and U.S. Marshals (USM) - the Branch Davidian raid in Waco, Texas, and the incident at Ruby Ridge. U.S. Attorney General Janet Reno under the direction of President Bill Clinton ordered the raid of the Branch Davidian complex and the incident at Ruby Ridge.
On November 25, 2002, the Homeland Security Act of 2002 split the Bureau of Alcohol, Tobacco and Firearms into two different parts.
The Bureau of Alcohol, Tobacco, and Firearms was renamed the Bureau of Alcohol, Tobacco, Firearms and Explosives and was moved into the Justice Department from the Treasury Department and is responsible for the law enforcement effort. It remains known officially by the acronym "ATF."
The Alcohol and Tobacco Tax and Trade Bureau (TTB) remained within the Treasury Department and continues to operate most of the tax collection side.
The changes took effect January 24, 2003.
ATF was originally formed from within the Internal Revenue Service (IRS) in 1973, though its history can be traced back to the time of the "Revenuers" and the Bureau of Prohibition. This is a name retained by the agency until the repeal of the Volstead Act in 1933. Special Agent Eliot Ness and his band of "Untouchables" were real-life members of ATF's predecessor organization. Often refered to as "treasury agents" then ATF agents.