What is an API?

An API (Application Programming Interface) is the agreed way for one program to ask another program for data or for an action. When you track a courier, check a PNR on IRCTC or pay through UPI, your app is not reading someone else's database directly. It sends a request to an API and receives a structured answer.

A menu, not a kitchen

An API is like a menu with a waiter. The menu lists exactly what you may ask for (the endpoints), the waiter carries your order to the kitchen (the server) and brings back a plate (the response). You never walk into the kitchen, so the restaurant can change its ovens without breaking your order.

What a request contains

A web API call is an ordinary HTTP request with four parts:

• a method saying what kind of action you want,
• a URL identifying the resource, such as https://api.example.in/v1/invoices/4821,
• headers carrying metadata such as Authorization and Content-Type,
• an optional body, usually JSON, holding data you are sending.

Method	Purpose	Example
GET	Read data	List all invoices for March
POST	Create	Add a new customer
PUT	Replace a whole record	Overwrite a product
PATCH	Update a few fields	Change only the price
DELETE	Remove	Cancel a draft invoice

A GET must never change anything on the server. If someone in your company builds GET /orders/12/delete, a browser prefetch or a crawler will eventually delete an order that nobody asked to delete.

Status codes

Every response carries a three-digit status code, and the first digit tells you where the problem is.

Code	Meaning	Typical cause
200 OK	Success	A normal GET
201 Created	Resource created	A successful POST
400 Bad Request	Malformed data	Missing field, bad GSTIN format
401 Unauthorized	No or invalid credentials	Expired token
403 Forbidden	Logged in but not allowed	Sales user hitting an admin endpoint
404 Not Found	Wrong URL or missing record	Typo in the invoice id
429 Too Many Requests	Rate limit hit	Looping without any delay
500 Server Error	Their bug	Retrying at once rarely helps

JSON, the common language

Nearly every modern API speaks JSON: objects of key/value pairs and arrays, which map neatly onto a Python dict and list.

{
  "invoice_no": "BP-2026-0481",
  "customer": "Sharma Sweets, Jaipur",
  "gstin": "08ABCDE1234F1Z5",
  "items": [{"name": "Kaju Katli 1kg", "qty": 12, "rate": 980.0}],
  "total": 11760.0,
  "paid": false
}

Responses are often nested several levels deep, so read the documentation (or print one response) to learn the shape before writing parsing code. Our JSON Formatter makes a long response readable while you explore it.

Where you will meet APIs

Payment gateways (Razorpay, UPI collect), GST return filing, logistics tracking, weather and market data, maps and pin-code lookups, WhatsApp and SMS sending, and every mobile app that talks to its own backend. Internally, one API often lets your billing system, your website and your Power BI report all read the same numbers.

Key takeaways

• An API is a contract: fixed endpoints, fixed request shape, predictable responses.
• The method says what you want, the URL says on what, the body carries the data.
• Always check the status code before you touch the body.
• JSON is the default data format; learn to read nested structures.