What is a REST API?

Imagine a restaurant.

You (the client) sit at the table.
The menu lists what you can order: burger, fries, milkshake.
The waiter takes your request to the kitchen (server), brings the food (data) back.

That’s what a REST API is:

You (the frontend or client app) make requests.
The server (kitchen) processes them.
The API (waiter) is how the two communicate.

REST just means:
“Let’s follow some simple rules to communicate over HTTP, using standard methods like GET, POST, PUT, DELETE.”

Core Concepts (And How to Explain Each One Simply)

1. Resources = Nouns

REST is about resources, like:

/books
/orders/123
/customers/5/orders

Each resource is represented by a URL.

2. HTTP Methods = Verbs

Think of CRUD operations:

Action	HTTP Method	What it does
Create	POST	Add a new item
Read	GET	Get item(s)
Update	PUT / PATCH	Modify item
Delete	DELETE	Remove item

You explain it like this:

“We use HTTP methods to tell the server what we want to do to a resource — like reading a book list (GET /books) or deleting a book (DELETE /books/3).”

3. Statelessness

Every request is independent.

“Imagine if the waiter forgot your last order. Every time, you have to tell them everything again. That’s stateless — REST APIs don’t remember anything between calls.”

4. JSON as the Common Language

Most REST APIs use JSON to send data back and forth.

“It’s like saying: we’ll all speak the same simple language — like JSON — to describe things like a book or a user.”

json
{
  "id": 1,
  "title": "Atomic Habits",
  "author": "James Clear"
}

How You Build a REST API in .NET

Let’s go step-by-step using an example:

Goal: Build a REST API to manage books.

Step 1: Create a Controller

C# Code
[ApiController]
[Route("api/[controller]")]
public class BooksController : ControllerBase
{
    private static List<Book> books = new List<Book>();

    [HttpGet]
    public ActionResult<List<Book>> GetAll() => books;

    [HttpGet("{id}")]
    public ActionResult<Book> GetById(int id) =>
        books.FirstOrDefault(b => b.Id == id);

    [HttpPost]
    public IActionResult Create(Book book)
    {
        books.Add(book);
        return CreatedAtAction(nameof(GetById), new { id = book.Id }, book);
    }

    [HttpPut("{id}")]
    public IActionResult Update(int id, Book updated)
    {
        var book = books.FirstOrDefault(b => b.Id == id);
        if (book == null) return NotFound();
        book.Title = updated.Title;
        return NoContent();
    }

    [HttpDelete("{id}")]
    public IActionResult Delete(int id)
    {
        var book = books.FirstOrDefault(b => b.Id == id);
        if (book == null) return NotFound();
        books.Remove(book);
        return NoContent();
    }
}

Now Practice Round: You Teach Me

Now try this, imagine I’m your junior teammate or interviewer. Teach me:

“What is a REST API and how does it work in C#?”

Give it a shot in your own words in the comments below, then I’ll give you friendly feedback and help you make it even clearer.