API (Application Programming Interface).

You can picture an API as the counter at a public office: there are set forms (the requests), fixed opening hours and responsibilities (the rules) and a dependable answer. What happens behind the counter you do not need to know. In the same way, a program offers certain functions through its API — 'give me all open orders', 'create a new contact' — and documents what the requests have to look like.

On the web, REST APIs are the most widespread. They work with ordinary HTTP requests to defined addresses and usually exchange data in JSON format. GET reads data, POST creates new data, PUT or PATCH changes it, DELETE removes it. There is also GraphQL, where the caller requests exactly the fields it needs. A webhook reverses the direction: instead of asking regularly whether anything new has happened, the other system calls an address as soon as an event occurs — a paid order, for example.

For SMEs the API is often the difference between a website that creates work and one that takes work away. Without an interface, an enquiry lands as an email in the inbox and is copied into the CRM by hand. With an interface, the form creates the contact directly, assigns the source and notifies the person responsible. The same applies to online shop and stock, booking system and calendar, or website and newsletter tool.

Interfaces come at a price: they have to be secured (access keys, never in front-end code), they can change when the provider releases a new version, and they need error handling for when the other system is unavailable. A solid integration logs what was transferred and retries after an outage instead of silently losing data.