A sentence is the legally binding words handed down by a judge in a court of law to a defendant, which dictates the punishment that will be meted out in that case.
Sentences can range in length and in means, such as incarceration only or incarceration plus a fine. Once a judge rules on a case and delivers the sentence as their verdict, it is immediately in effect.
Sentencing guidelines exist on a federal level for all cases that are decided on a state level. The presiding judge in the case has the ability to create a bespoke sentence that is within the acceptable range dictated within the guidelines. This is why different crimes coming before the courts can have different sentences. Judges have a tendency to give leniency to first-time offenders and deliver stricter sentences to habitual offenders.