An API (Application Programming Interface) is a defined contract that lets one software system request data or actions from another without knowing how the other system works internally. It specifies what you can ask for, how to ask, and what comes back.
What it is
An API is the agreed "menu" between two pieces of software. The provider publishes a set of operations — for example "get this customer," "create an order," or "cancel a subscription" — along with the exact format for requests and responses. The consumer calls those operations without ever seeing the provider's source code or database. That separation is the whole point: each side can change internally as long as the contract stays the same.
Why it matters
APIs are what make modern software composable. Almost nothing today is built entirely in-house: payments, email, maps, authentication, analytics, and AI are all consumed through APIs. A clean API lets teams integrate systems, swap vendors, expose a product to partners, and build mobile and web front ends on top of the same back end. Without APIs, every integration would mean sharing databases or copy-pasting data — brittle, insecure, and slow.
How it works: common API types
REST is the most widespread style. It exposes resources at predictable URLs (endpoints) and uses standard HTTP verbs — GET to read, POST to create, PUT/PATCH to update, DELETE to remove — typically returning JSON. It is simple, cacheable, and easy to debug.
GraphQL exposes a single endpoint where the client asks for exactly the fields it needs in one query. This avoids over-fetching and under-fetching, which is useful for rich, nested data and for mobile apps that want to minimise round trips.
Webhooks invert the model. Instead of your app repeatedly polling a service to ask "anything new?", the service pushes an HTTP request to a URL you provide the moment an event occurs — a payment clearing, a file uploading, a build finishing. This makes event-driven integrations near real-time.
Other styles you may meet include gRPC (fast, binary, common between back-end services) and SOAP (older, XML-based, still found in enterprise and banking systems).
In practice: example uses
A storefront calls a payment provider's API to charge a card; a logistics app calls a maps API for routing; a SaaS dashboard calls an internal REST API that its own mobile app and website both share. APIs also power microservices, where an application is split into small services that talk to each other over the network through well-defined API contracts. The same discipline — clear contracts, versioning, and authentication — applies whether the API is public, partner-facing, or purely internal.
Related terms
Explore related entries: GraphQL and Microservices, or browse the full glossary. If you are building or integrating systems, see Application Development.
FAQ
What is an API in simple terms?
An API is a set of rules that lets two software systems talk to each other. One side asks for data or an action in an agreed format, and the other side responds, without either side needing to know the other's internal code.
What is the difference between REST and GraphQL?
REST exposes many fixed endpoints that each return a set shape of data, while GraphQL exposes one endpoint where the client asks for exactly the fields it needs in a single query. REST is simpler and widely cached; GraphQL reduces over-fetching for complex, nested data.
What is a webhook?
A webhook is a reverse API call: instead of you polling a service for changes, the service sends an HTTP request to your URL the moment an event happens, such as a payment succeeding or a build finishing.
A well-designed API is the difference between systems that integrate cleanly and systems that fight each other — it is the contract everything else is built on.
At Apex IT Solutions, we design and integrate REST, GraphQL, and webhook-based APIs as part of standard delivery. Get a free consultation on your project.
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