A contract is a "promise" or an "agreement" made of a set of promises. The law generally sees performance of a contract as a duty and breach of this contract gives rise to legal remedies.

For a contract to be valid, it must meet the following criteria:

Mutual agreement: There must be an express or implied agreement. The essential requirement is that there be evidence that the parties’ conduct showed that they had assented to the contract.

Consideration: There must be consideration given by all the parties, meaning that every party is conferring a benefit on the other party or himself sustaining a recognizable detriment, such as a reduction of the party's alternative courses of action where the party would otherwise be free to act with respect to the subject matter without any limitation. Consideration need not be adequate, e.g. agreeing to buy a car for a penny may constitute a binding contract.

Competent, Adult Parties: Both parties must have the capacity to understand the terms of the contract they are entering into, and the consequences of the promises they make. For example, animals, minor children, and mentally disabled individuals do not have the capacity to form a contract, and any contracts with them will be considered void or voidable. Although corporations are technically legal fictions, they are considered persons under the law, and thus fit to engage in contracts.

For adults, most jurisdictions have statutes declaring that the capacity of parties to a contract is presumed. An adult asserting the failure of a contract because of incapacity has to prove that he or she was incapable in order to escape liability.

Proper Subject Matter: The contract must have a lawful purpose. A contract to commit murder in exchange for money will not be enforced by the courts. It is void ab initio, meaning "from the beginning."

Mutual Right to Remedy: Both parties must have an equal right to remedy upon breach of the terms by the other party

Mutual Obligation to Perform: Both parties must have some obligation to fulfill to the other. This can be distinct from consideration, which may be an initial inducement into the contract.

Intention to create legal relationship: There is a strict presumption for commercial agreements to be legally bound. Domestic and social agreements are usually unenforceable.

Unilateral vs. bilateral contracts

Contracts may be bilateral or unilateral. The more common of the two, a bilateral contract, is an agreement in which each of the parties to the contract makes a promise or promises to the other party. For example, in a contract for the sale of a home, the buyer promises to pay the seller $200,000 in exchange for the seller's promise to deliver title to the property.

In a unilateral contract, only one party to the contract makes a promise. A typical example is the reward contract: A promises to pay a reward to B if B finds A's dog. B is not obliged to find A's dog, but A is obliged to pay the reward to B if B finds the dog. In this example, the finding of the dog is a condition precedent to A's obligation to pay.

An offer of a unilateral contract may often be made to many people (or 'to the world') by means of an advertisement. In that situation, acceptance will only occur on satisfaction of the condition (such as the finding of the offeror's dog). If the condition is something that only one party can perform, both the offeror and offeree are protected — the offeror is protected because he will only ever be contractually obliged to one of the many offerees; and the offeree is protected, because if she does perform the condition, the offeror will be contractually obliged to pay her.

In unilateral contracts, the requirement that acceptance be communicated to the offeror is waived. The offeree accepts by performing the condition, and the offeree's performance is also treated as the price, or consideration, for the offeror's promise.

The most common type of unilateral contract is the insurance contract. The insurance company promises to pay the insured a stated amount of money on the happening of an event if the insured pays premiums; note that the insured does not make any promise to pay the premiums.

Courts generally favor bilateral contracts. The general rule in the United States is: "In case of doubt, an offer is interpreted as inviting the offeree to accept either by promising to perform what the offer requests or by rendering the performance, as the offeree chooses." Restatement (Second) of Contracts § 32 (1981) (emphasis added). Here the law attempts to provide some protection from the risk of revocation in a unilateral contract to the offeree. Note that if the offer specifically requests performance rather than a promise, a unilateral contract will exist.

Oral contracts

Contrary to common wisdom, an informal exchange of promises can still be binding and legally as valid as a written contract. Courts in the United States have generally ruled that if the parties have a meeting of the minds (i.e., the same intent), consideration is paid or given by the parties, and they act as though there was a formal, written and signed contract, then a contract exists. However, most jurisdictions require a signed writing for certain kinds of contracts (like real estate transactions).

When a contract is not binding

In general, there are three classifications of contracts that are not binding:

Void: If a contract is held to be void, the contract has never come into existence. For example, a contract is void if it is based on an illegal purpose or contrary to public policy; the classic example is a contract with a hit man. Such a contract will not be recognized by a court, and cannot be enforced by either party.

Voidable: A contract is voidable if one of the parties has the option to terminate the contract. Contracts with a minor are examples of voidable contracts.

Unenforceable: If a contract is unenforceable, neither party may enforce the other's obligations. For example, in the United States, a contract is unenforceable if it violates the Statute of frauds. An example of the above is an oral contract for the sale of a motorcycle for US$5,000 (because in the USA any contract for the sale of goods over US$500 must be in writing to be enforceable).

If the terms of the contract are uncertain or incomplete, the parties cannot have reached an agreement in the eyes of the law. An agreement to agree does not constitute a contract, and an inability to agree on key issues, which may include such things as price or safety, may cause the entire contract to fail.

However, a court will attempt to give effect to commercial contracts where possible, by construing a reasonable construction of the contract.

If there are uncertain or incomplete clauses in the contract and all options in resolving its true meaning have failed, it may be possible to sever and void just those affected clauses if the contract includes a severability clause. The test of whether a clause is severable is an objective test - whether a reasonable person would see the contract standing even without the clauses.

Express and implied contracts

A contract can be either an express contract or an implied contract. An express contract is one in which the terms are expressed verbally, either orally or in writing. An implied contract is one in which some of the terms are not expressed in words.

An implied contract can either be implied in fact or implied in law. A contract which is implied in fact is one in which the circumstances imply that parties have reached an agreement even though they have not done so expressly. For example, by going to a doctor for a physical, a patient agrees that he will pay a fair price for the service. If he refuses to pay after being examined, he has breached a contract implied in fact.

Quasi-contract

A contract which is implied in law is also called a quasi-contract, because it is not in fact a contract; rather, it is a means for the courts to remedy situations in which one party would be unjustly enriched were he or she not required to compensate the other. For example, an unconscious patient treated by a doctor at the scene of an accident has not agreed (either expressly or by implication) to pay the doctor for emergency services, but the patient would be unjustly enriched by the doctor's services were the patient not required to compensate the doctor.

Course of dealing

If two parties have regularly conducted business on certain terms, it may be reasonable to presume that in future dealings where there is no contract, the parties wish to incorporate the terms of the previous contracts. This practice is called “course of dealing” and refers the parties back to their prior contractual obligations.

Statutory law applicable to contracts

The rules by which many contracts are governed are provided in specialized statutes that deal with particular subjects. Most countries, for example, have statutes which deal directly with sale of goods, lease transactions, and trade practices. For example, most American states have adopted Article 2 of the Uniform Commercial Code, which regulates contracts for the sale of goods.

Remedies

Typically, the remedy for breach of contract is an award of money damages. Courts usually adopt one of three ways of calculating the value of damages.

The most common is to assess the sum which would restore the injured party to the economic position that he or she expected from performance of the promise or promises (known as an "expectation measure" or "benefit-of-the-bargain" measure of damages).

When it is either not possible or desirable to award damages measured in that way, a court may award money damages designed to restore the injured party to the economic position that he or she had occupied at the time the contract was entered (known as the "reliance measure"), or designed to prevent the breaching party from being unjustly enriched ("restitution").

Specific performance

There may be circumstances in which it would be unjust to permit the defaulting party simply to buy out the injured party with damages. For example where an art collector purchases a rare painting and the vendor refuses to deliver, the collector's damages would be equal to the sum paid.

The court may make an order of what is called "specific performance", requiring that the contract be performed. In some circumstances a court will order a party to perform his or her promise (an order of "specific performance") or issue an order, known as an "injunction," that a party refrain from doing something that would breach the contract.

Procedure

In order to obtain damages for breach of contract or to obtain specific performance, the injured party may file a civil (non-criminal) lawsuit, usually in a state court, or petition a private arbitrator to decide the contract issues presented.

Many contracts provide that all contract disputes must be arbitrated by the parties to the contract, rather than litigated in courts. By law, some contracts, including most securities brokerage contracts, must be arbitrated; other contracts are referred by courts as a matter of local law or policy. Arbitrated judgments are generally enforced and appealed in the same manner as ordinary court judgments; a majority of states have adopted the Uniform Arbitration Act to facilitate the enforcement of arbitrated judgments.

http://en.wikipedia.org/wiki/Contracts