An au pair is a live-in mother's help, working around 25 - 30 hours a week. They are not supposed to have long hours of childcare, nor placed in charge of a baby.
I'm a SAHM, so my au pair's main duties are daily cleaning and taking my girls to school and back. She also does babysitting at night.
The au pair goes to English classes (sorry if I am offending anyone by generalising) - mine goes three mornings a week. We are not expected to teach her English other than just chatting with her. She is not expected to teach the children anything as she is not in solo charge of them.
A lot of people find that adding an au pair into their family is a lot like getting another child to look after, but it doesn't have to be like that. Mine is 26, so definitely an adult, although she is not nearly as street-smart or world-aware as your average 26 year old here.