What is an API?

An API (Application Programming Interface) is an interface through which software components communicate. In a web context, web APIs let frontends, apps, or external services fetch or send data from a backend. The two main approaches for web APIs are REST and GraphQL—both typically return JSON.

An API follows a request–response pattern: the client (browser, app, another server) sends a request to a defined endpoint. The server processes the request and returns a response—usually JSON. The API defines which endpoints exist, which parameters are allowed, and what shape the response has. APIs run over HTTP(S) and use status codes for feedback (e.g. 200 OK, 201 Created, 400 Bad Request, 401 Unauthorized, 404 Not Found, 500 Server Error).

• Endpoints:Every API has URLs (endpoints), e.g. /api/users or /api/products. The client calls those URLs and receives data back.
• Authentication:Many APIs require an API key, JWT, or OAuth to grant access. Public APIs may work without authentication.
• Versioning:APIs evolve—versioning (e.g. /api/v1/users) enables backward compatibility. Older clients keep working; newer ones use extended endpoints.
• Documentation:Good APIs are documented—endpoints, parameters, examples. Tools like OpenAPI (Swagger) or GraphQL Playground make development easier.

REST (Representational State Transfer) is an architectural style for web APIs. Each resource (e.g. user data, products, orders) has its own URL. The HTTP method defines the action: GET (read), POST (create), PUT/PATCH (update), DELETE (delete). REST is widely adopted, easy to understand, and works well with caching, proxies, and CDNs.

• Resource-oriented:URLs represent resources—/users/123 for a user, /orders for orders. Nested resources: /users/123/orders.
• Stateless:Each request contains everything needed. The server does not store session state between requests—scaling and caching become simpler.
• JSON:REST APIs usually return JSON. XML is an alternative. Content-Type and Accept headers control the format.

GraphQL is a query language and specification for APIs. Unlike REST, there is typically a single endpoint. The client sends a query and requests exactly the fields it needs—avoiding overfetching (too much data) and underfetching (multiple round trips). GraphQL fits well for complex data structures and flexible client needs.

• Schema:GraphQL defines a type system—which objects, fields, and relationships exist. The client knows the structure and can query precisely.
• Queries, mutations, subscriptions:Queries read data, mutations change data, subscriptions enable real-time updates—all via one endpoint.
• Introspection:GraphQL APIs can describe themselves—tools like GraphQL Playground or Apollo Studio support exploratory testing.

Both approaches have strengths. REST is widespread, easy to learn, and supported by many tools. GraphQL offers more flexibility in data fetching and reduces round trips. The choice depends on the project.

• Choose REST when:Simple CRUD, established infrastructure (caching, CDN), many clients with similar needs. Many CMS and e-commerce systems expose REST APIs.
• Choose GraphQL when:Complex nested data, many clients with different data needs, real-time features (subscriptions). Headless CMS and modern mobile apps often use GraphQL.
• Hybrid:Some systems offer both—REST for simple cases, GraphQL for complex queries.

APIs connect systems: websites fetch content from a backend; mobile apps use the same API as the web app. External services—payments, email, maps—offer APIs for integration. Modern architectures separate frontend and backend; the API is the bridge.

• Internal APIs:Communication between your own systems—e.g. frontend and backend of a web app. Often not public.
• External APIs:Public or partner-specific interfaces—e.g. Stripe for payments, Google Maps, weather services. Often require API keys and rate limits.
• Headless:A backend (e.g. headless CMS) only delivers data via API; the frontend (web, app, IoT) decides how to present it.

APIs are the backbone of modern applications—they connect frontend and backend, enable integration with external services, and support cross-platform architectures. REST and GraphQL are the leading approaches; the choice depends on project, data model, and team experience.

IVIS MEDIA develops APIs for websites, web apps, and cross-platform applications—from REST and GraphQL to integration with existing systems. More about web development.