"Shift work" is when you work the same hours on the days you work, but the hours may not be the typical daytime hours. For example, a factory might have three shifts each day: 8:00 to 16:00, 16:00-24:00, 24:00- 8:00. If your schedule changes from day to day, I'd say you "don't have a fixed schedule." This means some days you might work 8:00-16:00 and others 12:00-20:00, just as an example. This sort of schedule is common in part-time work, but you can also work part-time on a fixed schedule.