Open API and OpenAPI: What's the Difference?

Open API and OpenAPI are two rather different things and ideas, but they do get confused sometimes. If you are looking for an explanation, this is the video for you. We'll talk about APIs in general, how the idea of an "Open API" fits right into the general idea of what APIs are and what they should do, and then we'll talk about how OpenAPI as a description format allows you to describe a certain type of API (resource-oriented APIs that use HTTP).
APIs in general are about the idea of providing networked and reusable capabilities. One way to look at them is to see them as digital building blocks. An API makes a promise that it can be used in a certain way, and that can be seen as a language how to interact with that API. Depending on the API style, this language works differently, but the general idea is always the same.
Open APIs then are the idea that these building blocks should be openly available. This doesn't mean that they are always available to everyone, the audience still can be limited. Distinguishing between public, partner, and private APIs is a common way of creating three levels of "openness".
OpenAPI is a standardized format to describe APIs. It works best for HTTP APIs that are using the resource style, meaning that the main abstraction of the API are URI-identified resources. The API then defines which HTTP methods it accepts, which responses it may give, and which data formats (typically JSON) are being used in this conversation.
This allows us to pull Open APIs and OpenAPI together: Ideally, API management works with the idea of a catalog where all APIs of an organization are managed. Populating this catalog with API descriptions and consuming APIs from the catalog then needs some way of describing the APIs in the catalog. OpenAPI is a good foundation for describing those APIs that are HTTP-based and resource-oriented. For other APIs, API description languages such as WSDL, GraphQL, or AsyncAPI may be better choices. But as you can see, the idea of open API works well with OpenAPI, so these two things are complementary rather than alternatives.

OpenAPI Initiative: https://www.openapis.org/
OpenAPI Specification: https://spec.openapis.org/oas/latest.html

00:00 Introduction
00:17 Short Version
01:27 What's an API?
02:40 What's an Open API?
04:46 How to use Open APIs
06:44 What is OpenAPI?
07:38 OpenAPI Example
08:34 How to use OpenAPI for Open APIs
10:08 Summary and Wrap-up
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