The Secret Service was commissioned on July 5, 1865 in Washington, D.C, to suppress counterfeit currency, which is why it was established under the Department of the Treasury. After the assassination of President William McKinley in 1901, Congress informally requested Secret Service Presidential protection. A year later, it assumed full-time responsibility for protection of the President.
The reason for this combination of duties is that when the need for presidential protection became apparent in the late 19th century, there were only a few federal services with the necessary abilities. The FBI, CIA, BATF, and DEA did not yet exist. The United States Marshals Service was the only other logical choice, and in fact the U.S. Marshalls did provide protection for the president at some points.
The Secret Service Presidential Protection Detail safeguards the President of the United States and his immediate family. They are heavily armed and work with local police and the military to safeguard the President when he travels.
The Secret Service has 5,000 employees: 2100 special agents, 1200 Uniformed Division employees, and 1700 technical and administrative employees. The Special Agents are the ones who bodyguard officials and investigate financial fraud. Uniformed officers provide security at the White House and Treasury building and other sites.
Like other federal law enforcement organizations, the Service has its critics. Such critics may point, for example, to an incident where Steve Jackson Games was raided by (perhaps overzealous) Secret Service agents in a move that was later ruled to be illegal and unjustified. The Secret Service has also been involved in investigations, arrests, and detentions that were allegedly motivated by political issues rather than security concerns. For instance:
In 1968, as a result of Presidential candidate Robert F. Kennedy's assassination, Congress authorized protection of major Presidential and Vice Presidential candidates and nominees. (Public Law 90-331). Congress also authorized protection of widows of Presidents until death, or remarriage, and their children until age 16.
Congress passed legislation in 1994 stating that Presidents elected to office after January 1, 1997, will receive Secret Service protection for 10 years after leaving office. Individuals elected to office prior to January 1, 1997, will continue to receive lifetime protection. (Public Law 103-329)
The Service also investigates forgery of government checks, forgery of currency equivalents (such as travelers' checks), and certain instances of wire fraud (such as the so called Nigerian "419" advance fee scheme) and credit card fraud.
The Service and the FBI each see themselves as the most prestigious and capable federal law enforcement agency. There is some animosity between the two organizations, and very few agents have served in both.