The DOJ often investigates Federal Criminal cases for years before bringing charges. This means that Federal charges may be completely put together before the person even knows he or she was even under investigation.
Federal Sentencing Guidelines are no longer mandatory, but they still take most discretion away from judges. At best, they give a judge an often high starting point in a sentencing decision. At worst they provide for sometimes harsh mandatory minimum sentences, especially the enhancer for having a gun during the commission of a crime.
The FBI investigates White Collar Crime, including Bank Fraud and Postal Fraud, as well as Bank Robbery and Hobbs Act, or Robbery cases with interstate commerce implications.
The DEA investigates larger drug cases. Most of these cases involve larger-scale drug distribution conspiracies. Many people are convicted in Federal Court without ever being found with drugs on their person.
The IRS investigates various Tax Fraud Cases, including filing fraudulent returns using fake ID numbers, as well as filing fraudulent personal returns which significantly understate the income and/or assets of an individual.