Welcome to the fascinating world of APIs! An API (Application Programming Interface) is a set of rules that allow two applications to communicate with each other. Think of it as a restaurant menu. When you go to a restaurant, you look at the menu and order your food. The kitchen, or the system, then prepares your order and serves it to you. In this case, the menu is akin to an API. It describes what is possible, and the kitchen uses those instructions to prepare your order.

So, how does this apply to web development? Let's take the example of a weather application that shows weather data from different cities. The app fetches this data from a weather API. The app sends a request to the API, similar to placing an order from a menu, and the API returns the requested data, similar to the kitchen serving the food.

One of the core concepts of REST is the use of HTTP methods on resources. These methods are:

1. GET: Retrieves data. It doesn't change the state or data on the server; it just retrieves it.
2. POST: Sends data to be processed to a specified resource. This method can change the state and the data on the server.
3. PUT: Updates some data on the server.
4. PATCH: Partially updates some data on the server.
5. DELETE: Deletes some data from the server.

You already know how to create simple routes in Express. However, building a REST API involves creating routes that correspond to the different HTTP methods. For example: