Certain contracts are not enforceable unless they are memorialized in a signed written agreement.  This is called the Statute of Frauds.  There are exceptions to the general rule, including instances when the writing may be pieced together from various things like emails.  Another exception is where the parties have already partially performed their obligations.  This area of law can be complex and is often hotly contested in commercial litigation.

General Obligations Law § 5-701
(As of 9/20/15 – please check for the current statute as the law may change over time)

Agreements required to be in writing

a. Every agreement, promise or undertaking is void, unless it or some note or memorandum thereof be in writing, and subscribed by the party to be charged therewith, or by his lawful agent, if such agreement, promise or undertaking:

1. By its terms is not to be performed within one year from the making thereof or the performance of which is not to be completed before the end of a lifetime;

2. Is a special promise to answer for the debt, default or miscarriage of another person;

3. Is made in consideration of marriage, except mutual promises to marry;

4. Repealed

5. Is a subsequent or new promise to pay a debt discharged in bankruptcy;

6. Notwithstanding section 2-201 of the uniform commercial code, if the goods be sold at public auction, and the auctioneer at the time of the sale, enters in a sale book, a memorandum specifying the nature and price of the property sold, the terms of the sale, the name of the purchaser, and the name of the person on whose account the sale was made, such memorandum is equivalent in effect to a note of the contract or sale, subscribed by the party to be charged therewith;

7, 8. Repealed

9. Is a contract to assign or an assignment, with or without consideration to the promisor, of a life or health or accident insurance policy, or a promise, with or without consideration to the promisor, to name a beneficiary of any such policy. This provision shall not apply to a policy of industrial life or health or accident insurance.

10. Is a contract to pay compensation for services rendered in negotiating a loan, or in negotiating the purchase, sale, exchange, renting or leasing of any real estate or interest therein, or of a business opportunity, business, its good will, inventory, fixtures or an interest therein, including a majority of the voting stock interest in a corporation and including the creating of a partnership interest. “Negotiating” includes procuring an introduction to a party to the transaction or assisting in the negotiation or consummation of the transaction. This provision shall apply to a contract implied in fact or in law to pay reasonable compensation but shall not apply to a contract to pay compensation to an auctioneer, an attorney at law, or a duly licensed real estate broker or real estate salesman.