The general rule of contract law is that the parties to a contract must stand in privity to one another. Privity means that both parties must have a legally recognized interest in the subject of the contract if they are to be bound by it. Outside parties who do not have such an interest in the subject matter of the contract may not be bound by it. Their right to sue in the event of breach (i.e., broken or violated) of contract would also be called into question. An exception to the general rule of privity exists in cases involving warranties and product liability.

Contractual characteristics are divided into four different categories:

• valid, void, voidable, and unenforceable;

• unilateral and bilateral;

• express and implied; and

• informal and formal.

Any given contract could be classifiable in all four ways. Thus, a single contract could be said to be valid, bilateral, express, and formal.

A valid contract is one that is legally binding and fully enforceable by the court. In contrast, a void contract is one that has no legal effect whatsoever. A contract to perform an illegal act would be void. A voidable contract is one that may be avoided or canceled by one of the parties. A contract made by minors and one that is induced by fraud or misrepresentation are examples of voidable contracts. An unenforceable contract is one that, because of some rule of law, cannot be upheld by a court of law. An unenforceable contract may have all the elements of a complete contract and still be unenforceable.

A unilateral contract is an agreement in which one party makes a promise to do something in return for an act of some sort. In contrast, a bilateral contract is one in which both parties make promises. A bilateral contract comes into existence the moment the two promises are made. A breach of contract occurs when one of the two parties fails to keep the promise.

A contract can be either express or implied. An express contract requires some sort of written or spoken expression that indicates the desire of the parties to enter the contractual relationship. An implied contract is created by the actions or gestures of the parties involved in the transaction.

In some situations, laws require certain types of contracts to be in writing. A written contract does not have to be a long, formal, preprinted agreement. A written contract may take the form of a letter, sales slip and receipt, notation, or memorandum. A written contract may be typed, printed, scrawled, or written in beautiful penmanship.

One who knowingly accepts benefits from another person may be obligated for their payment, even though no express agreement has been made. Agreements of this type can be either implied in fact or implied in law.

Contracts implied by the direct or indirect acts of the parties are known as implied-in-fact contracts.

An implied-in-law contract can be imposed by a court applying reasons of justice and fairness when someone is unjustly enriched at the innocent expense of another. It is used when a contract cannot be enforced or when there is no actual written, oral, or implied-in-fact agreement. An implied-in-law contract is also called a quasi-contract. It does not result from the mutual assent of the parties such as an express or implied-in-fact contract.

Under common law principles, a formal contract differs from other types in that it has to be written; signed, witnessed, and placed under the seal of the parties; and delivered.

A special type of formal contract – contract of record –