Let me give you a very easy-to-understand example.

One day, Wheel Bro created an AI that specializes in crawling spicy Zhihu articles. Every day, he checks the list and likes them. Coincidentally, you are also a fan of those articles. In this case, Wheel Bro’s account is your API interface. The only thing you need to do is follow his account—then you can simply read his daily posts to see the content, without caring how he actually finds so many articles.

Wheel Bro, please don’t hit me~~ (つд⊂)

Friends who think about this example will probably understand from the definition: an API is an interface—a channel responsible for communication between one program and other software. In essence, it is a set of predefined functions. Many people have already given intuitive examples. Here, I’d like to explain what makes a good API from another angle.

For example, when we go to handle administrative affairs, the service counter is similar to an API. If for a complicated matter the counter can achieve “only one visit” and “only one stamp,” then that API is well designed. (Of course, APIs are slightly different and follow the Interface Segregation Principle—for example, user registration and user login should be two separate interfaces, which improves system flexibility.)

But in reality, “only one visit” is still difficult to achieve. It requires internal departments to clearly organize workflows and data channels so that the service window can easily obtain all required information. This shows that designing a truly good API is not easy.

Here is another explanation from a designer’s perspective, emphasizing that APIs contain computation and logical judgment:

Assume that in logistics, “goods” are data, the “central warehouse” is the database, and the “shops” are websites or apps. The content shown on pages, numbers, user actions, requests, and results are all “goods” that must be constantly transported—data. The transfer station responsible for dispatching, allocating, and packaging these goods is the API. The shop staff only need to retrieve goods from this station.

From this, the role of APIs becomes clear:

- For software providers: by offering APIs for other applications to call, an ecosystem is formed. The software can realize its maximum value and gain stronger vitality—while the source code remains hidden and business secrets stay protected.

- For application developers: open APIs allow them to directly use mature services from multiple companies instead of building everything from scratch, greatly saving time and effort.

In the era of cloud computing and the sharing economy, APIs are the way technology service providers deliver services to customers. For example, NetEase Cloud builds communication and video services based on over ten years of IM experience. Developers can quickly implement powerful messaging and video features by integrating client SDKs and cloud Open APIs.

When talking about APIs, they are often mentioned together with SDKs.

So what exactly is an API? Just look at how you order food in a restaurant.

You arrive at the restaurant, call the waiter, and order food.

The waiter brings you the menu. Whatever you order, she writes it down.

After ordering, she sends the list to the kitchen.

Here, the waiter is the service provider—otherwise she wouldn’t be called a “waiter.”

What service does she provide?

Food-ordering service.

What does this service require?

When discussing a service, we usually talk about inputs and outputs.

In this example, the input is the dish name (or dish ID—some menus use numbers only), and the output is the prepared dish brought to your table.

With defined input and output, the waiter can provide the ordering function. This is an API. The customer is the caller, and the waiter is the service provider.

You could even replace the waiter with a cat. Suppose Her Majesty the Cat serves you—if the input is dish names and the output is dishes, then the API works perfectly fine.

And all customers can order food this way.

Now think again—haven’t you seen waiters using handheld ordering devices? A beautiful waitress holding a device about the size of a smartphone. That device needs to communicate with the kitchen system. This interaction requires constraints that define what the input is and what the output is. For example, if a dish is sold out, the system must notify the customer.

APIs are usually provided in HTTP form. The hidden meaning is simple: as long as you follow the defined rules, you can use the service.

For example, the waiter is Chinese and the customer is American. No problem—as long as the American speaks Chinese, the API works. If the American only speaks English, what then? Let the Chinese friend translate—problem solved.

So what is an SDK?

If the American can’t speak Chinese, the restaurant manager comes over to translate. That manager is the SDK. SDKs are usually language-specific and are official implementation packages provided for different programming languages.

Now let’s broaden our perspective.

Besides HTTP APIs, do internal system components also have APIs?

The answer is yes. For example, the various APIs provided by the JDK itself. In this case, the distinction between API and SDK becomes less clear, but the core meaning of an API remains the same: when a service provider exposes functionality, it must clearly define the input, output, and behavior.

A collection of clearly defined inputs, outputs, and functional descriptions constitutes the APIs provided by the service.

For example, the methods exposed by arrays or linked lists.

So is there a clear difference between APIs and methods?

Publicly exposed methods that can be used by others are collectively called APIs.

This explanation may not be academically rigorous, but for beginners, it should be sufficient.

If you find APIs difficult to understand, the problem is probably not the API itself—but whether you truly understand what encapsulation is, and what a service really means.