An API (Application Programming Interface) is a defined way for one piece of software to ask another for data or to trigger an action. It works like an agreed contract: the requesting program knows what it may ask for and in what format, and the providing system decides what it will answer, without either side needing to see the other's internal code.
A Checkout Page as an Example
When a customer pays on an online store, the website does not process the card itself. Its server sends a request to a payment gateway's API containing the order amount and a secure payment token. The gateway checks the payment and sends back a response saying whether it succeeded or failed. The store reads that response and shows a confirmation or an error message. The same pattern powers shipping-rate lookups, SMS and WhatsApp notifications, embedded maps and sign-in with a Google account.
Anatomy of an API Request
Most web APIs follow the same basic shape:
- Endpoint: the URL that represents a resource, such as an orders or customers address
- Method: the action requested. GET reads data, POST creates it, PUT or PATCH updates it and DELETE removes it
- Authentication: an API key or token proving the caller is allowed to ask
- Response: usually JSON data plus a status code, where 200 signals success and 404 or 500 signal problems
Good providers publish documentation describing each endpoint, the fields it expects and the errors it can return.
REST, SOAP and GraphQL
REST is the most common style on the web. It uses ordinary HTTP methods and URLs, and typically returns JSON. SOAP is an older, stricter protocol that uses XML and still appears in banking and enterprise systems. GraphQL lets the caller specify exactly which fields it wants in a single request, which can reduce the number of round trips for complex screens.
APIs also differ by audience. Public APIs are open to any developer, private APIs are used only inside one company, and partner APIs are shared under an agreement.
Most calls happen when your system asks. Some services also push information to you: a webhook is a message a provider sends to your server when something happens, such as a payment being confirmed. It saves your system from repeatedly asking whether anything has changed, a pattern known as polling, and it makes updates arrive faster and with less wasted traffic.
What to Check Before Integrating One
- Rate limits that cap how many requests you can make per minute or per day
- Pricing, since some APIs charge per call
- Versioning, and how long older versions stay supported
- How errors are reported, and what your site shows visitors when the service is unavailable
- Where API keys are stored. They belong on the server, never in public browser code
- What personal data is shared, and whether that fits your privacy obligations
Because an integration depends on a service you do not control, a sensible build includes timeouts and fallback behaviour. Applications that connect a website with CRM tools, billing systems or mobile apps are usually planned as part of custom software development. The server-side code that sends and receives these requests is covered under backend development, and the browser-side counterpart is frontend development.