There are several classifications of APIs based on how they are used:
These APIs are used exclusively within an organization or company. AlthoughÂ adoption of REST is growing strong among all industries within an enterprise,Â SOAP/HTTP or .NET still has a large presence. In most cases, REST willÂ wrap an existing SOAP/HTTP or .NET service.
These APIs are primarily available externally to consumers. At this stage ofÂ maturity, the growing trend for external APIs are written based onÂ REST/JSON technologies. They provide access and integration capabilitiesÂ that are easier to use than the more industrial-strength capabilities leveragingÂ web services (for example, WSDLs).
These APIs are specifically designed for partners to be able to accessÂ business functions in relation to the business relationship of the partnership.Â Examples include online catalog, ordering, and reconciliation.
Over time, other categories of APIs might come into being.
To learn more please read the RedbookÂ here.