According to Section 3 of the 25th Amendment, during a period in which the president believes he will be temporarily unable to dicharge his powers, he may declare himself "unable to discharge the powers and duties of his office," at which point the Vice President discharges the duties of the office as Acting President.
Section 4 of the amendment also permits the Vice President, together with a majority of the members of the President's cabinet, to declare the President incapacitated. This provision has never been invoked to date.
The Acting President has the authority to execute all the constitutional powers of the presidency until his congressionally approved mandate expires. At that point, the real president resumes his office.
During his period in power, the Acting President is treated with the same protocol as any other president, even being addressed as "Mr. President" by White House staff.
Only twice thus far in American history have we had an "Acting President." In both cases, power was transferred voluntarily by the President as provided for under Section 3 of the amendment:
see also: acting