An API (application programming interface) is a set of rules that lets one program ask another program for data or an action, and get an answer back. When a delivery app shows a map, takes a card payment or lets you sign in with Google, it’s using APIs. The app doesn’t contain a map or a bank. It asks the services that have them.
For anyone building a product, APIs decide a lot: how fast you can add features, what you pay every month and where your data goes.
API stands for Application Programming Interface. It’s the part of a program that other programs are allowed to talk to. It lists what you can ask for, what you need to send and what you get back. Everything else inside the program stays hidden.
When you open a web page or a mobile app, it talks to a server through that server’s API. The server receives the request, does the work and sends a response. That’s how you can pay, book or upload something without leaving the app.
Many companies now design their software around APIs. In Postman’s 2025 State of the API report, a survey of over 5,700 developers, architects and executives, 82% of organizations had adopted some level of an API-first approach, and 25% were fully API-first. 65% said they make money from their APIs.
Every API call is a request and a response:
A simple way to picture it: a waiter in a restaurant. You order from the menu, the waiter takes the order to the kitchen and brings back your food. You never go into the kitchen, and you can only order what’s on the menu.
That hiding has a name: encapsulation. One program can use another without knowing how it was built or how it handles the data inside. Developers don’t have to read someone else’s code to connect to it, and fewer places in the system can break when one side changes.
An API works like a contract between two programs. For each function it defines three things:
An API can have one function or hundreds. Developers often group them: one set for sign-up and login, one for payments, one for reports. One standard way to write this contract down for web APIs is the OpenAPI Specification. Its latest version, OpenAPI 3.2.0, was published on 19 September 2025. A file in this format lets people and tools understand what an API does without reading its code.
On Traza, a booking app for tattoo artists, the API was exactly that kind of contract between two teams. The app talks to Traza’s live REST API. Traza’s team owned the backend, and ours built the mobile app on top of it.
If you’re building a digital product, APIs matter for five reasons:
Koncierge, a payment platform for Japan that we built, shows the last two points. Its core is the data and the API, with a thin interface on top. The pipeline from bank feed to matched deposit worked before any screen sat on it. Merchants connect once, through one merchant API, and get a webhook when a deposit is matched.
The most useful way to sort APIs for a business is by who can use them.
| Type | Who can use it | Example | What it means for you |
|---|---|---|---|
| Private (internal) | Only your own apps | The API between your mobile app and your backend | You control it fully; it’s part of your build cost |
| Partner | Selected companies under an agreement | A merchant API like Koncierge’s | You set the rules and support the partners who use it |
| Public (open) | Any developer, often with a key | Maps, payments, sign-in | Quick to add, but you depend on the provider’s prices and rules |
APIs also differ by style. Many web APIs are REST APIs that send JSON over HTTP. Others push data to you when something happens, which is called a webhook. Operating systems have their own APIs too: iOS and Android give apps access to the camera, location and notifications through them.
Google Calendar and Apple’s calendar let other apps create, change and delete events through their APIs. A clinic or a salon can let customers book an appointment that lands straight in the staff calendar, and the customer sees it in their own calendar on every device.
Most people don’t like long sign-up forms, so many apps offer “Sign in with Google” or “Sign in with Apple”. These buttons are APIs from the companies that run the accounts. There’s a rule to know for iPhone apps: if you offer a third-party or social login, Apple’s App Review Guidelines (4.8) require an equivalent login option that limits data to the user’s name and email, lets users keep their email private and doesn’t collect their activity for ads without consent.
Food delivery, taxis and running apps all show maps from Google, Apple or other providers through their APIs. These calls cost money past a free amount. On Google Maps Platform’s pricing page, Dynamic Maps and Geocoding each come with 10,000 free calls a month. After that, Dynamic Maps costs $7.00 and Geocoding $5.00 per 1,000 calls in the first paid tier. A small app stays free. A popular one gets a monthly bill that grows with its users, so plan for it.
A third-party API is somebody else’s product inside yours. Its prices, rules and uptime become your problem. On IPOPBOOKS, a bookkeeping platform for US small businesses, the first bank data provider asked for a stack of security policies before it would sign. We moved the bank feeds to another provider so the launch didn’t wait, and kept the first one in the code for later.
Security is the other half. The OWASP API Security Top 10 puts broken object level authorization first: an API that returns another user’s data when someone changes an ID in the request. In Postman’s 2025 survey, 51% of developers named unauthorized access by AI agents as a top security risk, now that agents call APIs too.
Before you rely on an API, check:
Integrations are a separate line in our estimates, because each one adds work: reading the provider’s documentation, handling errors and testing with real accounts. Our project estimate turns your brief into user stories. You confirm what is in scope, and every hour in the budget traces back to a story you kept. We check the hours the model suggests against what our team actually books on finished projects, then multiply by our published rate of $39-55 an hour.
Here is an example run of our estimate’s math with sample inputs: a mobile payments app built on a licensed payment provider, with an identity-check vendor. Connecting the payment provider and the identity-check vendor came to 28 of 260 hours. The whole budget came to $8,700 to $18,000, over 4 weeks. The inputs are assumptions, not a quote. To get the same breakdown for your product, see what your idea costs to build.
APIs are part of every product we build, from web apps to custom backends and integrations.
An API is a set of rules that lets one program ask another for data or an action and get an answer back. It’s like a menu: you can order what’s on it without going into the kitchen.
An app sends a request to the server’s API with what it needs and a key that shows who is asking. The server checks the request, does the work and sends back a response, usually as JSON, which the app shows to the user.
By who can use them: private APIs for your own apps, partner APIs for selected companies, and public APIs for any developer. By style, most web APIs are REST APIs, and webhooks push data to you when something happens.
Many public APIs are free up to a limit and then charge per call. Google Maps Platform, for example, gives 10,000 free Dynamic Maps loads a month and then charges $7.00 per 1,000 in the first paid tier. Building and testing each integration also takes developer hours.
It’s as safe as its checks. The first risk on the OWASP API Security Top 10 is an API that returns another user’s data when someone changes an ID. Every request should be checked for who is asking and what they may see.
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