API (application programming interface)
An API is a documented way for one system to ask another system for data or to send it something, and get an answer back immediately. On a website it is what lets a page show live stock, order status or pricing that lives in another system rather than a number somebody typed in.
In practice
The property that matters for a B2B website is timing. An API answers a question at the moment it is asked, which is what every customer-facing feature depends on. A customer portal showing order status, a page showing account-specific pricing, a quote form that creates a record in your ERP rather than an email in an inbox: all of those are an API doing its job while the page loads.
The second property is reach. Building the first integration is the expensive part, because getting data out of an ERP in a usable shape is genuinely hard work. Once it exists, the marginal cost of the next thing that reads it is small. That is why the sequence matters: companies that wire each system directly to each other end up maintaining several versions of the same business logic that disagree, and companies that build one internal interface first add the website, the portal and the next trading partner as additions rather than rebuilds.
Most modern APIs work over HTTP, the same protocol as the web itself, whose semantics are specified in RFC 9110 [1]. The shape of a particular API is usually described using the OpenAPI Specification [2], which is worth knowing because a vendor who can hand you an OpenAPI document is a vendor whose integration will take days rather than weeks.
When evaluating any software an established B2B company plans to keep, the question worth asking early is whether it has a documented API at all. A system without one is a system your data cannot leave without an export, which quietly decides what you can build for the next decade.
A test you can run this week: ask whoever supports your ERP whether it has a documented API, and ask for the documentation rather than a yes. If what comes back is a PDF describing a nightly file export, you do not have an API, you have a batch job, and any real-time feature on your website will need something built in between.
Common questions
What is an API in plain terms?
A defined way for two pieces of software to talk to each other. One system sends a request in an agreed format, the other sends back an answer. It is the difference between your website showing the stock figure that is true right now and showing one somebody updated last Tuesday.
Why does an API matter for our website?
Because everything a customer would log in to see depends on one. Order status, shipment tracking, account pricing and self-service quoting all require the page to ask another system a question while it loads. Without an API those features cannot exist, however good the website looks.
What is an API integration?
The work of connecting two systems through their APIs so data moves between them without anyone rekeying it. In a B2B context that usually means connecting an ERP or CRM to a website, a portal or an ecommerce platform. The effort is mostly in mapping the data, not in the connection itself.
Sources
- RFC 9110, HTTP Semantics rfc-editor.org/rfc/rfc9110
- OpenAPI Specification spec.openapis.org/oas/latest.html