In 1973, the IASC (International Accounting Standards Committee) was established. They issued IAS until 2000. They issued IAS 1 to 41.
In 2001 IASC was restructured to become the International Accounting Standards Board (IASB). IASB adopted all IAS and any standards issued further was named IFRS.
So all standards issued from 1973 to 2000 is known as IAS (iussed by IASC) and all standards issued from 2001 onwards (By IASB which was formerly IASC) is known as IFRS.
If there is a contradiction between IAS and IFRS, then IFRS will prevail. Certain IFRS replaces IAS.