What's the difference between calendar reminder types "alert" and "status bar notification"?
"Alert" will cause a new window with the event details (or a list of events with their info, if more than one) to pop up on top of the active one  and take focus when the reminder occurs. It also puts a notification icon in your status bar tray. The "status bar notification" setting causes the status bar icon to appear but doesn't automatically bring up the details dialog.