What distinguishes an API from a web service?

The distinguishing characteristic of an API as compared to a web service is that APIs can exist independently of web services. An API (Application Programming Interface) provides a set of rules and protocols for building software and applications, enabling different software entities to communicate with each other. This means that APIs can be implemented in various forms, including but not limited to web services.

Web services, on the other hand, are a specific subset of APIs that facilitate communication over the web and typically use standard web protocols like HTTP. Therefore, while all web services are APIs (since they also provide interfaces for programmatic access), not all APIs are web services, as APIs can also manifest in other forms, such as library APIs, operating system APIs, etc.

This fundamental understanding of the relationship between APIs and web services helps clarify that the flexibility of APIs to exist outside the realm of web services is what sets them apart.