The arraignment is a hearing in court in which the District Attorney charges a defendant with a specific offenses. The defendant receives a copy of the charges (in a document called a "complaint") and is informed of their constitutional rights. The defendant enters a plea of guilty or not guilty at this hearing.
The preliminary hearing is a hearing in court where both the District Attorney and the defense can present evidence, and a judge decides if the evidence adequately supports requiring the defendant to stand trial. A preliminary hearing is not a trial and does not involve a jury.
A pre-trial conference provides the opportunity for the prosecution and the defense to discuss the status of a case with the judge. Pre-trial conferences can result in plea agreements or the setting of a trial date.