Broadly speaking, law enforcement is the act of making citizens or subjects follow the law, by force if necessary. In the United States, enforcement of state laws is the job of state police, county sheriffs, and city police departments.
At the federal level, several different agencies handle law enforcement, with the Federal Bureau of Investigation being the closest thing the U.S. has to a national, general-purpose police force. Other federal law enforcement agencies are primarily responsible for enforcing a relatively small subset of federal laws.
For example, the Drug Enforcement Administration has the power to enforce federal laws covering illegal drugs, while the Bureau of Alcohol, Tobacco, Firearms, and Explosives enforces the laws related to those legal, but tightly regulated, products. The Secret Service, in addition to being responsible for protecting the President and other VIPs, also investigates the counterfeiting of U.S. currency.
Many law enforcement officers and officials are trained at law enforcement colleges, which teach students basic investigative skills, as well as other issues having to do with the criminal justice system and law enforcement.