An API (Application Programming Interface) is a collection of code that, rather than being run directly by the user as a program, is meant to be used by other programs. This is different from a function in that an API may contain many functions that perform related tasks; the API groups them together. It is a library of functions.
This list of API's are not officially part of ComputerCraft and act as extensions and/or utilities, and require manual installation. For the official list of API's, see the API list.