A pardon means that you are fully cleared of any responsibility for the crime for which you had been convicted–the conviction is gone. A commutation means that you still have a record and your conviction stands, but your sentence is changed.
The sentence changes to however the governor or president changes the sentence. The sentence can be a death sentence that is commuted to life, a life sentence that is commuted to a parole eligible sentence, or a jail sentence to a probationary sentence.
Often, a commutation is a very good outcome and much more reasonable request.