As described at OpenAPI-Specification:
The OpenAPI Specification (OAS) defines a standard and programming language-agnostic interface description for HTTP APIs, which allows both humans and computers to discover and understand the capabilities of a service without requiring access to source code, additional documentation, or inspection of network traffic. When properly defined via OpenAPI, a consumer can understand and interact with the remote service with minimal implementation logic. Similar to what interface descriptions do for lower-level programming, the OpenAPI Specification removes the guesswork in calling a service.
Use cases for machine-readable API definition documents include, but are not limited to: interactive documentation; code generation for documentation, clients, and servers; and automation of test cases. OpenAPI documents describe APIs services and are represented in YAML or JSON formats. These documents may be produced and served statically or generated dynamically from an application.
Swagger Editor offers an easy way to get started with the OpenAPI Specification.
You may find a more comprehensive list at OpenAPI Tooling.