계약 - legally binding contract of any kind. (You fill out and sign a 계약서 for any large transaction) 협약 - agreement reached through negotiation (between countries or official institutions. "협약을 맺다"). 약정 - commercial or political commitment (not as comprehensive as 계약 or 조약. "사용 조건을 약정하다"). 조약 - formal treaty between countries (베르사이유 조약 = The Treaty of Versailles). 차관 - financial aids provided between countries, usually with favorable payback terms ("한국은 1950년대에 미국으로부터 많은 차관을 들여왔다"). 계약 and 약정 may involve both individuals and institutions, and sometimes countries. 조약 and 차관 are almost always between two or more countries. 협약 is a somewhat loose term that only apply to countries and institutions. (For ordinary people, we say 약속).

계약 its about business or rent house ex) private*private , samsung * bmw 협약 its about group ex) seoul * la , seoul * samsung 약정 when you contract phone, tv service. period or charge 조약 country *country ex) korea * uk 차관 be rent money or rent money with country