What is API?
An API (Application Programming Interface) is a set of rules and definitions that lets one piece of software talk to another. It specifies the requests a client can make, the data formats to use, and the responses to expect — hiding the internal implementation behind a stable contract.
On the web, "API" usually means an HTTP API that returns data (often JSON) over endpoints. APIs power everything from mobile apps to integrations between services.
Key points
- Defines a contract: available operations, inputs, and outputs.
- Hides internal implementation behind a stable interface.
- Web APIs typically use HTTP and exchange JSON.
- Common styles include REST, GraphQL, and gRPC.
Example
GET https://api.example.com/users/1
→ { "id": 1, "name": "Ada" }Common uses
- Connecting a frontend to a backend
- Integrating third-party services (payments, maps, auth)
- Exposing data to mobile and partner apps
- Automating workflows between systems