What is an API?
By Yassine Seghrouchni, founderUpdated
In short
An API (application programming interface) is the fixed entry point a software package opens up to other software. Where a person logs in and clicks buttons, another system sends a request: give me today’s invoices, or create this customer. The API replies with the data or a confirmation. That’s how systems work together without anyone retyping anything.
Also known as: application programming interface
How an API works
An API is a set of fixed requests a system understands. The vendor describes them in the API documentation: which data you can read, which you can change, in what format you send it and how many requests you can make per minute.
With most modern packages, it’s a REST API: requests over the web, with the data usually in JSON. Each kind of data has its own address, an endpoint, for instance for customers, orders or invoices. To get in, you need a key or token, so the system knows who is asking and what they’re allowed to do.
An example: with the KVK’s APIs, you look up a company in the Dutch business register and get back its name, KVK number and branch address. A form or CRM that uses that API fills in those details by itself. More under KVK integration.
API, integration and webhook
An API does nothing on its own. It’s the entry point of a single system. An API integration is the software that connects the APIs of two systems: it fetches data from one, maps it and writes it into the other.
A webhook works the other way round. With it, a system sends a signal on its own when something happens, such as a new order. The integration then fetches the details through the API.
AI works through APIs too. An MCP server usually calls the API of the system it opens up, and an AI agent reaches your CRM or accounts through APIs.
Why it matters in practice
Look at the API when you choose software, not only when you want to integrate it. A package without a usable API locks in your data: you can only get at it through an export or by retyping. So ask the vendor three questions. Is there an API, and is it publicly documented? Can you read and change all the data you use with it, or only the basics? And does API access cost extra, or is it included in your subscription?
A common misconception: “the package has an API, so integrating it will be quick”. An API tells you which door there is, not what’s behind it. Sometimes exactly the fields you care about are missing, or you can only make a limited number of requests. Read the documentation before you decide. Why the cost of an integration varies so much is covered in what an integration between two systems costs.
Frequently asked questions
What does API stand for?
API stands for application programming interface. In plain terms: the entry point a software package opens up to other software. Through it, another system requests data or gives an instruction, without a person in between.
What is the difference between an API and an API integration?
The API is the entry point of a single system and comes from the vendor. The integration is the software that connects two of those entry points: it fetches data from one system, maps it and writes it into the other. Someone has to switch on or build that integration.
Does every software package have an API?
No. Most modern packages do, but what you can do with it varies widely. Some APIs only give access to part of the data, or only on a more expensive plan. Check before you choose a package.
