Many offenses are prosecuted at the state level, and federal level. If the federal government files charges, it's possible that state prosecutors will dismiss their state criminal charges, or delay them, in order to allow the federal prosecutors to take over. The great majority of trials involve prosecutions. Federal crimes are offenses described by the United States Code - this is a document of laws passed by Congress. Title 18 of the USC is the portion of law which deals with investigating and prosecuting crimes. There a few agencies which handle this, such as the FBI, Secret Service, ATF, Department of Homeland Security, and others. Federal charges carry harsher penalties than state charges. The reason is that these indictments usually involve severe consequences. The majority of convictions for federal charges and crimes result in federal prison time.