Exchange students are most likely to be high school students. Exchange students live abroad for up to one year, and may live with more than one host family in a homestay during his/her stay.
Exchange students are expected to maintain decent grades, abide by the rules of host families and the laws of host countries.
Exchanges are facilitated by for-profit companies, charitable organizations like Rotary International, and between schools or "sister cities." A fee is almost always associated. Host families are not generally compensated, although a small stipend (perhaps $50 / month) may be paid.
Exchange students, either through personal resources or through the entity facilitating the exchange, are expected to obtain medical insurance, spending money and emergency funds, though the facilitating entity may have emergency contingency plans.
Exchange students are, above all, expected to have a wonderful year, semester or summer abroad, immersing themselves in new cultures and seeing the world from a new viewpoint.