The term "au pair" is a French word and means something like "in equality". In common usage today it describes a foreign national, usually between the ages of 19 to 27 who provides childcare and some household duties in exchange for living with another family (usually abroad) and some educational benefits. It has been called a cultural and language experience, among other things, including an extended vacation abroad. 

The aupair concept originated in Europe and has recently become more popular in the United States. Less expensive air travel has been a factor in this rising popularity, as has increased demands for in-home childcare. In the case of most countries the concept has been furthered by the difficulties of its youth in finding work after completing high school and college studies. For some it's a year's vacation/experience before tackling the "real world's" job market. (Weather could be a factor, too, just think about those long, cold, dark Scandinavian winters!) 

Ideally, the au pair is treated as a family member, but is also an employee in most sense of the word. Typical work schedules average 40 to 50 hours per week, with evenings and weekends off but vary considerably with the needs of the individual family. Typical salaries are $115 to $150 per week; a private room, bathroom, board, and other amenities are provided, as well as car insurance, and an option to drive the family's car occasionally. Some (like the USIA and the eight so-called "legal au pair" programs) like to call this salary a "stipend", in order to avoid the obvious, that they are dealing with an employer/employee relationship rather than a family member relationship. In most cases the above issues are handled on a case by case basis between the family and the au pair.

Anyway, that is what's an au pair.