At the most superficial level, before you see user interfaces and programs, an operating system is all API. Computer source code is the blueprint of a computer program. That is not an analogy. Computer source code is an actual, tangible blueprint. Computer source code is not a program, it is the blueprint for a program. The blueprint describes what a program is supposed to do when it actually runs and what the program will take in as data and give out as data. Each part of the blueprint has names. The blueprint is the full design of the program with the names of the parts within the blueprint. The organization of names forms a pattern, a unique fingerprint for the program. That unique fingerprint is the API.