After a defendant enters a plea of guilty or no contest to a criminal charge (or a DUI charge), a trial court judge often refers the defendant to the local probation department so that the probation department can conduct a "presentence investigation" on the defendant. After all, most times most judges want as much relevant material they can read about the defendant before they pronounce what they believe is a fair sentence. If the defendant is out on bond, the defendant must walk almost immediately to the probation department to be interviewed and have his/her background investigated by the probation department. If the defendant is still in jail (because he/she did not make bond), the probation officer will come to them. Once the presentence investigation report is complete, it will be sent to the Judge, and, often (but not always - depends on the Judge) the attorney for the defendant will get to review the report. One of the most important things a defendant can do during this process is to be fair and honest with the probation department (although, if a defendant intends on appealing his guilty verdict, sometimes the defendant will want to continue his/her right to remain silent). How the probation officer perceives a defendant often goes a long way toward what they Judge will think of the defendant on sentencing day. Defendants who smirk or appear indifferent to the probation officer will often pay for that attitude on sentencing day (in one way or another).
So be nice - very nice - to your probation officer. They may be just a clerk to you - but they are really little gods with a pen.