To summarize, we can group web APIs into four broad categories:

Open APIs, which any developer can access.

Partner APIs, which only authorized developers may access.

Internal APIs, which only internal teams may access.

Composite APIs, which combine multiple APIs.

There are also three common types of API architectures:

REST, a collection of guidelines for lightweight, scalable web APIs.

SOAP, a stricter protocol for more secure APIs.

RPC, a protocol for invoking processes that can be written with XML (XML-RPC) or JSON (JSON-RPC).

https://blog.hubspot.com/website/application-programming-interface-api