The federal Sentencing Guidelines were established to create uniformity of sentences for different individuals charged with similar conduct. The guidelines were written by the United States Sentencing Commission and then adopted by Congress. Originally, the Sentencing Guidelines were mandatory. Judges were required to impose a sentence within the sentencing range specified by the Guidelines.
That all changed in 2005, when the United States Supreme Court handed down it’s opinion in United States v. Booker, and struck down the mandatory aspect of the Guidelines. Instead of being mandatory, application of the Sentencing Guidelines is merely advisory. Now a judge, after consulting the Guidelines, is free to tailor a sentence based on an individuals situation.