"This report provides an introduction to the roles that international law and agreements play in the United States. International law is derived from two primary sources--international agreements and customary practice. Under the U.S. legal system, international agreements can be entered into by means of a treaty or an executive agreement. The Constitution allocates primary responsibility for entering into such agreements to the executive branch, but Congress also plays an essential role. First, in order for a treaty (but not an executive agreement) to become binding upon the United States, the Senate must provide its advice and consent to treaty ratification by a two-thirds majority. Secondly, Congress may authorize congressional-executive agreements. Thirdly, many treaties and executive agreements are not self-executing, meaning that implementing legislation is required to provide U.S. bodies with the domestic legal authority necessary to enforce and comply with an international agreement's provisions."
CRS Report for Congress, RL32528