When a promise is reduced to the form of a written agreement under seal, it is called a covenant. And when the promise is conditional, the condition must be performed before it becomes of binding force.
Promises are express or implied.
This is the principle, but what amounts to a promise is not so easily defined. In general a promisee can maintain an action on a promise made to him, but when the consideration moves not from the promisee, but some other person, the latter, and not the promisee, has a cause of action, because he is the person for whose use the contract was made.