Русско-англо-русский юридический словарь

A written contract as the name suggests, is a contract whose terms have been reduced to writing. Written contracts are also commonly signed. However, a written contract may consist of an exchange of correspondence, a letter written by the promisee and assented to by the promisor without signature, or even a memorandum or printed document not signed by either party. (definitions.uslegal.com)

The bilateral agreement involves two parties each promising to do something. The parties can be individuals, groups, businesses or governments. Somehow, the two actions are mutually supportive, binding and inclusive. Both parties fulfill roles as the promisor and the promisee. (thelawdictionary.org)

A unilateral agreement is where one party makes a promise, declaration or offer. For example, a radio station DJ tells the audience that he will pay a listener $100 if he can answer a question. The radio station is the promisor and the listener is the promisee. The two sides of the agreement are not equal or mutual. The promisor offers money and the promisee must offer an answer. (thelawdictionary.org)