IAS stands for International Accounting Standards. The IAS has a lot of sub-entities, such as the IAS Board, which is the major authority that sets its standards. Many people adhere to both the IASB and IASC, not just for influence but also for what they stand for on accounting ideals. GAAP or Generally Accepted Principles is the more Americanized term referring to the accounting standards present in any country.
The GAAP determines the rules or standards, as well as the principles, followed when one summarizes, records, and prepares financial statements within the nation. It does not possess as many sub-entities as IAS. IASC does not directly influence or set the rules for GAAP. FASB is in charge of dictating the actual accounting rules, which will, later on, become the GAAP for the country.