A "statute of frauds" requires that certain contracts be in writing, and that they be signed by all parties to be bound by the contract. Although there can be significant variation between jurisdictions, the most common types of contracts to which a statute of fraud applies are:
Contracts involving the sale or transfer of land;
Contracts to answer for the debt or duty of another;
Contracts that, by its terms, cannot be completed within one year. (Please note that the fact that a contract is not completed within one year does not mean that it is voidable under a statute of frauds. For the statute to apply, the actual terms of the contract must make it impossible for performance to be completed within one year); and
Certain contracts for the sale of goods, under the Uniform Commercial Code.
Typically, to satisfy the requirements of the statute, the writing must identify the contracting parties, recite the subject matter of the contract such that it can reasonably be identified, and present the essential terms and conditions of the parties' agreement. (Under the Uniform Commercial Code, to satisfy the statute, the writing for the sale of goods need only be signed by the party to be charged, and a quantity term.)
Please note that, even without respect to the Statute of Frauds, it is good practice to reduce the essential terms of any contract to a signed, written agreement. Even when a Statute of Frauds does not apply to an oral contract,1 it may be very difficult to prove and enforce the contract in the absence of a written agreement.
The purpose of a "statute of frauds" is, as the name suggests, to prevent injury from fraudulent conduct. There is some criticism of the continued existence of these statutes, as they are often used by parties who freely entered into fair contracts yet wish to avoid having to fulfill their agreements. At the same time, the abuses these statutes were designed to prevent are quite real, so a strong argument remains to keep them in place. It is also arguably good public policy to require that parties to certain significant transactions, such as those of long duration or which involve real estate, reduce their agreements to writing. A writing will both reduce the chance of future litigation, and also give the parties the opportunity to take a second look at the terms and conditions of their agreement before it becomes final.
A statute of frauds does not of itself render a contract void. The statute makes certain contracts "voidable" by one of the parties, in the event that the party does not wish to follow through on the agreement. (A contract that is "void" cannot be enforced. A contract that is "voidable" remains valid unless one of the parties chooses to void the contract.)
Sometimes, a party to a contract that would otherwise be invalid under a "statute of frauds" will nonetheless be able to enforce it, on the basis of "partial performance" or "promissory estoppel". Where "partial performance" exists, a party who has accepted partial performance by another party under the contract will typically be barred from asserting the "Statute of Frauds" in order to avoid meeting its own contractual obligations. "Promissory estoppel"2 exists where significant inequities (unfairness) would result from releasing a party from the contract, and the party seeking release knew or reasonably should have known that those inequities would be created at the time of the original agreement. For example, where the party which seeks to be released knew that the other party would incur significant expense in obtaining materials which cannot be transferred to other work, a court may find that under the circumstances the contract should be enforced despite the statute of frauds.
As previously noted, if all parties agree that they are bound by the contract, the contract will remain enforceable despite the statute of frauds.
 Sometimes the phrase "verbal contract" is used to describe an unwritten or "oral" contract. As one meaning of "verbal" is "in words", in order to avoid any ambiguity it is usually best to refer to unwritten contracts as "oral contracts".
 The elements of promissory estoppel are typically:
A clear and definite offer;
Reasonable expectation of reliance on the offer;
Reasonable reliance on the offer by the party receiving the offer; and
Detriment as a result of reliance on the offer.