Quasi contracts are obligations that are imposed by law even if the basic elements of a valid contract are not present. They arise due to principles of justice and equity rather than an agreement between the parties. Examples include necessaries provided to incapable persons, payment made on another's behalf, benefits enjoyed from a non-gratuitous act, responsibilities of finders of goods, and money paid by mistake or under coercion. The key idea is that no person should unjustly enrich themselves at another's expense.