Passed by Congress July 6, 1965. Ratified February 10, 1967.
Article 2 Section 1 had only specified that if the President was unable to discharge his duties, the office of President "shall devolve upon the Vice President." It did not clarify whether the Vice President became the President with the full authority of the Office or the Acting President who does the President's job. The 25th Amendment clarified this. The Vice President would become President if for some reason the President was unable to discharge their duties.
The 25th Amendment only provides for the Vice President succeeding the President in case of something happening to the President. The Presidential Succession Act of 1947 supplements this and provides for a more complete order of succession to the President of the United States. (During a State of the Union Address when President, Vice President, Supreme Court Justices and Cabinet Officers are in one place, one Cabinet Officer does not attend the Address. He is the "Designated Survivor" in case something happens to the others.) Here is the order of succession: