API stands for American Petroleum Institute (API).
API has been developing standards for the industry since 1924. Originally, standards were developed to assure that equipment was compatible. For example, a standard was developed so that the threads on the end of a section of pipe would match up with any other section of pipe, regardless of who manufactured it.
It depends on the type of API thread (there are several)
API line pipe threads are identical (and will mate with) in most respects to NPT with the exception that there is a counterbore in the female or "box" that covers the partial threads on the male or "pin". there are also NUE and EUE and Buttress threads that API cover as well.