The Iran-Contra Affair (also sometimes called Iran-gate) was the result of an arms deal between the United States, Iran and the Nicaraguan Contras, an anti-communist guerilla group operating against the Marxist government of Nicaragua. Essentially, arms were sold to Iran against an international embargo which was spearheaded just years earlier by the United States. The proceeds from this arms sale were then used to purchase additional arms for the Contras operating in the mountains of Nicaragua.
The scandal was first discovered by an assistant to Attorney General Ed Meese, who found a memorandum written by Lieutenant Colonel Oliver North, which documented the diversion of funds to the Contras. After the discovery of the memorandum, President Ronald Reagan met with both houses of Congress to disclose their findings; he then asked for the appointment of an independent counsel.
In the opinion of independent counsel Lawrence E. Walsh, who investigated the affair, under the Arms Export Control Act  such sales to foreign powers not deemed under the term of the act friendly are illegal.  Payments to the Contras from the proceeds of private sales, while not illegal, in the opinion of critics were considered highly unethical. Later such arrangements were made explicitly illegal by the Boland amendment to the Federal Appropriation Bill. The independent counsel concluded that the Reagan administration misled congressional investigators and withheld documents: "large volumes of highly relevant, contemporaneously created documents were systematically and willfully withheld from investigators by several Reagan Administration officials" and "following the revelation of these operations in October and November 1986, Reagan Administration officials deliberately deceived the Congress and the public about the level and extent of official knowledge of and support for these operations."
The week after the story was blown open by media outlets in the wake of midterm congressional elections after Democrats retook the Senate in November 1986, President Reagan returned to the airwaves to affirm that weapons were transferred to Iran, but he claimed they were not part of an exchange for hostages. In a speech in March of 1987, he acknowledged that the arms were in fact exchanged for hostages, saying that "what began as a strategic opening to Iran deteriorated, in its implementation, into trading arms for hostages."  The Reagan administration cooperated with the investigation, and no wrongdoing was ever proven. Reagan later stated in his autobiography that, "until Ed Meese uncovered North's memorandum, I had not heard a whisper about funds being channeled from the Iranian arms shipments to the Contras, and I would have not approved of it if anyone had suggested it to me....Yes, I believed in helping the Contras; but no one, including the President, is above the law." 
Michael Ledeen was an early player in the affair, and eventually arranged for arms sales to Iran though the Iranian arms dealer Manucher Ghorbanifar. John Poindexter later resigned, but returned to work under George W. Bush's administration. Oliver North was also a major player in the affair and was called upon to testify before Congress and was questioned for a number of days. His strong answers in the face of Congressional grilling led him to become a type of folk hero at Congress's expense. Caspar Weinberger was indicted, and pardoned by George H.W. Bush.
- The CIA-Contra-Crack Cocaine Controversy: A Review of the Justice Department's Investigations and Prosecutions, December 1997
- An American Life, The Autobiography of Ronald Reagan, pg. 530
- An American Life, The Autobiography of Ronald Reagan, pg. 542