A quasi-contract is a back-to-back agreement between two parties who have never had any prior commitments to each other. A judge creates it to rectify a situation in which one party gains something at the expense of the other.
The contract attempts to prohibit one party from gaining an unfair advantage at the cost of the other. When a party accepts products or services while not having requested them, these agreements may be imposed. The acceptance then establishes a payment expectation.