An API (short for Application Programming Interface) is a set of rules and protocols that allows different software applications to communicate with each other. In other words, it is an interface that allows one application to access functionality or data provided by another application or service.

When an application wants to access the data or functionality of another piece of software (or hardware), it cannot do so randomly. It must know what to ask for, how to ask, and what to expect in response. The API provides exactly this information: it describes what requests are allowed, in what format they must be made, and what you will receive in return. For example, if you want a map service to show you the nearest restaurants, the map service API will say: “To get this data, send me a request containing geographic coordinates and the type of place you are looking for, and I will respond with a list of results.”

Generally if a website or application provides an API, it will also provide an API Documentation in a section dedicated to explaining how to interact with it. Within the documentation you will find the exact syntax for requesting data through the API.

From a technical point of view, APIs operate through standardized communication protocols, such as HTTP/HTTPS for Web APIs. These protocols define how the client (the software that makes the request) and server (the software that provides the response) interact. For example, an API may accept requests with specific methods such as GET (to get data), POST (to send data) PUT (to update data) or DELETE.

Data are often exchanged in formats such as JSON or XML, which are readable by both machines and humans.

A request might look like this:

GET /weather?location=Rome&unit=celsius

And the API response could be:

{
  "location": "Rome",
  "temperature": "12",
  "unit": "celsius"
}

How to use an API

To understand how to use an API, we can start with a practical, concrete example: getting weather information via the OpenWeather API.

The basic idea is that we will send a structured request to the OpenWeather server, providing all the necessary details, such as the name of the city and the format in which we want to receive the information. In response, the API will return the requested data, organized in a format that is easily understood and usable by our system.

We need to imagine the request to the OpenWeather API as being a well-formatted, properly addressed letter. The letter might say something like, “Dear OpenWeather, can you tell me the weather for Rome? I would prefer to know the temperature in degrees Celsius.” In technical terms, this request could be written like this:

GET https://api.openweathermap.org/data/2.5/weather?q=Milan&units=metric&appid=YOUR_API_KEY

Here the system uses the GET method, which is like saying, “I want to read or get information.” It also specifies some essential information: the name of the city (q=Milan), the unit of measure for temperature (units=metric for degrees Celsius), and finally a special access key (appid=YOUR_API_KEY) that identifies who is making the request. To request an API key, you must register on the OpenWeather site.

OpenWeather, riceve questa richiesta da parte dell'API, la legge, e poi risponde con i dati richiesti. La risposta è formattata in JSON potrebbe essere simile a questa:

{
  "coord": { "lon": 12.48, "lat": 41.89 },
  "weather": [
    { "id": 800, "main": "Clear", "description": "clear sky", "icon": "01d" }
  ],
  "main": {
    "temp": 15.5,
    "feels_like": 13.9,
    "temp_min": 14.0,
    "temp_max": 17.0,
    "pressure": 1018,
    "humidity": 67
  },
  "name": "Milan"
}