An API (application programming interface) is the set of rules by which one piece of software lets another read from it or write to it. API integration is using those rules to connect systems so data moves and actions happen without a person copying anything. The website form that lands in your CRM with its source attached is an API integration working; the lead that arrives in an inbox and waits is one that is missing.
This guide is part of our technology guides. The most common business integration, the CRM, is covered step by step in CRM implementation steps; the systems being connected are often the web applications described in web app vs mobile app.
What an API is
MDN Web Docs, in its introduction to web APIs (last modified 4 September 2026), describes APIs as constructs made available in programming languages that allow developers to create complex functionality more easily, abstracting the more complex code away and offering simpler syntax in its place. In business terms: an API is the counter at which one system serves another. You ask in an agreed form, you get an answer in an agreed form, and you never see the kitchen.
MDN also distinguishes the APIs built into a browser from third-party APIs, which are not built in and whose code and information are retrieved from somewhere on the web. Its examples are the ones most businesses already use without naming them: the Google Maps API to show a map on a site, the YouTube API to embed and search videos, the Twilio API for calls and text messages. Each is a service that other software can use through its API.
What API integration is
API integration is connecting two or more systems through their APIs so that data flows or actions happen automatically, for a defined business purpose. It has three parts: a mapping (which field in system A becomes which field in system B, and what to do when they do not match), the calls (software that asks one API and tells the other, on a schedule or when an event happens) and the handling (what happens when a call fails, a record is missing or an API changes). The mapping is where most of the thinking is; the handling is where most integrations are let down.
How an API works, in Stripe's words
Stripe's API reference describes its API in terms that fit most business APIs you will meet. It is organized around REST; it has predictable, resource-oriented URLs (one address per kind of thing, such as a customer or a payment); it accepts form-encoded request bodies and returns JSON-encoded responses; and it uses standard HTTP response codes, authentication and verbs. Two further points matter for anyone commissioning an integration. The API key used to authenticate decides whether a request runs in live mode or in a sandbox, so integrations can be built and tested without touching real money or data. And the API does not support bulk updates: one object per request, which is why moving ten thousand records is a job that is scheduled and monitored.
How an API integration is built: five steps
The five steps our API integration service runs, and that any good integration follows:
- Map the data. List every field that moves, in which direction, and what the two systems call it. Decide the matching key (an email, a domain, an order number), what happens on a mismatch, and which system is the truth for each field.
- Check the APIs. Read both systems' documentation: what can be read and written, how authentication works, the rate limits (how many calls a minute), whether the API pushes events (webhooks) or must be asked (polling), and how versions change. Many APIs publish a machine-readable description in the OpenAPI format, which the OpenAPI Initiative's specification (version 3.2.1, 10 September 2026) describes as a standard, language-agnostic interface description for HTTP APIs that lets people and computers understand a service's capabilities without its source code or further documentation.
- Build and test in sandboxes. Write the calls and the handling, run them against the test modes both systems offer, and test the failures deliberately: a missing field, a duplicate, a timeout, a rate limit hit.
- Go live with monitoring. Switch to live keys, and watch: every failed call logged and alerted, every record reconciled against its source for the first weeks.
- Look after it. APIs change versions, fields get renamed, keys expire. Someone owns the integration, reads the alerts and tests after every change on either side.
The kinds of API and integration you will meet
The table below sets out the kinds a business meets most, with an example of each. In short: REST and other HTTP APIs are what you call when you want something; webhooks are what a system calls when something happens; file exchanges move batches on a schedule; and third-party service APIs (maps, payments, messaging, video) add a capability to your own software without building it.
Examples a business will recognize
Six integrations most companies either have or need:
- Website form to CRM. A visitor submits an inquiry; the integration creates or updates the contact in the CRM, with the page, campaign and source attached, and assigns an owner. The most common and most useful small integration; our guide to CRM implementation steps places it in the wider project.
- Taking payment. A shop or SaaS product creates a payment through a provider's API such as Stripe's, which handles the card, the bank and the receipt, and tells your system the result through a webhook.
- Showing a map. A location page shows an interactive map and directions through the Google Maps API, the example MDN uses for a third-party API.
- Sending a text or a call. An appointment reminder goes out through a messaging API such as Twilio's, named by MDN, from your booking system.
- CRM to accounting. A deal marked won becomes a customer and an invoice in the accounting system, with no retyping and one record of the truth.
- Product catalog to marketplace. Stock levels and prices from the inventory system update the online store and the marketplaces several times a day, by API rather than by spreadsheet upload.
Connector tools or custom code?
No-code connector platforms can link popular tools in minutes and suit simple, standard flows: a form to a mailing list, a new deal to a chat message. They cost a monthly fee per volume of actions and show what they are doing in a visual editor. Custom code suits the rest: when the data must be transformed or matched with judgment, when volumes are high enough that per-action pricing or rate limits bite, when one of the APIs is unusual or private, when failures must be handled with care (money, bookings, compliance), or when the integration is part of a web application you own. Many companies run both: connectors for the simple flows, custom integrations for the ones the business depends on. Our web application development team builds the second kind into the applications it delivers.
What goes wrong, and what to ask for
The six failures behind most broken integrations, and the question that prevents each:
- Silent failures. A call fails and nobody knows until a customer complains. Ask for logging, alerts and a weekly reconciliation.
- Two truths. The same field edited in both systems, with each overwriting the other. Ask which system owns each field.
- Rate limits. The API refuses calls beyond a quota and records are dropped. Ask how the integration queues and retries.
- Shared passwords. A person's login used as the integration's identity. Ask for dedicated API keys with the least access needed, stored outside the code and rotated.
- Version changes. The API provider retires a version and the integration stops. Ask who watches the provider's change notices.
- No test mode. Built straight against live data. Ask that it is built and tested in sandboxes first, as Stripe's keys allow.
Conclusion
API integration is connecting systems through the defined ways they let other software use them, so that data moves and actions happen without people retyping. It is built by mapping the data, checking the APIs, building and testing in sandboxes, going live with monitoring and looking after it as the APIs change. Connector tools handle the simple flows; custom code handles the ones the business depends on. The questions to ask are about failures, ownership of fields, limits, keys and versions, because that is where integrations break.
PromotEdge International is an international strategy, technology, marketing and design company, and our delivery team is in India. Our API integration service connects websites, CRMs and business applications this way, and our CRM implementation team makes the CRM the system the others feed.
Facts we rely on: see our company facts for what we state about ourselves and where each fact comes from.