What is a Treaty?
Treaties are a serious legal undertaking both in international and domestic law. Internationally, once in force, treaties are binding on the parties and become part of international law. Domestically, treaties to which the United States is a party are equivalent in status to Federal legislation, forming part of what the Constitution calls "the supreme Law of the Land." However, the word treaty does not have the same meaning in the United States and in international law. Under international law, a "treaty" is any legally binding agreement between nations. In the United States, the word treaty is reserved for an agreement that is made "by and with the Advice and Consent of the Senate" (Article II, Section 2, Clause 2 of the Constitution). International agreements not submitted to the Senate are known as "executive agreements" in the United States, but they are considered treaties and therefore binding under international law.
Source: Treaties and Other International Agreements: The Role of the United States Senate: A Study (prepared for the Committee on Foreign Relations, United States Senate, S. Print 106-71, (Washington, DC: U.S. Government Printing Office, 2001), p. 1.