A mother's helper is usually there to help with the kids while at least one parent is still present: maybe taking care of lunch and activities while Mom works from home, or doing the laundry while Dad gets the kids to bed. Nannies work on a more fixed schedule, either part-time or full-time, and there are usually employment/tax considerations to keep in mind. Au pairs come from other countries and live with the family, watching the kids and tending to other household responsibilities in exchange for room/board and sometimes a monetary stipend. Compared with those three situations, babysitting is much less formal and more of an as-needed childcare arrangement.