What is an API?

An API, or Application Programming Interface, is a contract that tells one system how to request information or action from another system.

That contract defines the available operations, the data a client must send, the data it receives back, and the errors it may need to handle.

Web APIs

When most people think of APIs, they think of web services.

Web services are APIs that let software communicate over the internet. A mobile app can request data from a server, a browser can submit a form, or two backend systems can exchange updates.

An API is broader than web services. It is any defined interface that tells one system how to ask another system for information or action. Good APIs make the available operations, inputs, outputs, and error cases predictable.

What APIs are used for

APIs are used for many product and business workflows, including:

• Real-time communication (chat, video, streaming updates).
• Data access and reporting.
• Notifications and messaging.
• Payments and subscriptions.
• Identity and access control.

What makes an API useful

There are many ways to design an API for each use case. Three things matter most: consistency, reliability, and documentation. Security belongs in reliability because an API is not dependable if it is not safe to use.

Many standards exist for those challenges. You can learn and apply them directly, work with specialists, or use software such as Proto Studio to create APIs with consistent structure and documentation from the start.

Next step

To understand why companies invest in APIs as products, read What is the API economy?.