How different between "WebSocket" and "REST API"

You can provide a REST API along with a WebSocket API for different purposes. It's up to your requirements and it depends on what you want to achieve.

For example, a WebSocket API can be used to provide real-time notifications while the REST API can be used to manage your resources.

There are a few details you should be aware of:

• REST is a protocol independent architectural style frequently implemented over the HTTP protocol and it's supposed to be stateless.
• WebSocket is a bi-directional, full-duplex and persistent connection protocol, hence it's stateful.

Just to mention one example of application that provides different APIs: Stack Exchange provides a REST API along with a WebSocket API.

A REST API uses HTTP as the underlying protocol for communication, which in turn follows the request and response paradigm. However, with WebSockets, although the communication still starts off over HTTP, it is further elevated to follow the WebSockets protocol if both the server and the client are compliant with the protocol (not all entities support the WebSockets protocol).

Now with WebSockets, it is possible to establish a full duplex and persistent connection between the client and a server. This means that unlike a request and a response, the connection stays open for as long as the application is running, and since it is full duplex, two way simultaneous communication is possible i.e now the server is capable of initiating a communication and 'push' some data to the client.

This is the primary concept use in the realtime technology where you are able to get new updates in the form of server push without the client having to request (refresh the page) repeatedly. Examples of such applications are Uber car's location tracking, Push Notifications, Stock market prices updating in realtime etc.

Here's a video from a presentation I gave earlier this month about websockets and how they are different than using the regular REST APIs: https://www.youtube.com/watch?v=PJZ06MLXGwA&list=PLZWI9MjJG-V_Y52VWLPZE1KtUTykyGTpJ&index=2

I haven't yet fully understood what a REST API is, but I guess you refer to it in a broarder way so as to web systems that provide structured data referd to a specific resource as can be a customer, or a product, upon a POST or GET call over http.

The main difference from a practical and simplistic approach is that HTTP GET / POST are request - response protocols. The server will send a response upon a request sent by the client.

In the case of Websockets, communication is bidirectional. Either the server or the client can send information to the counterpart at any time.

To visualize the difference a page that provides stock market data if using HTTP GET will issue a new request to the server each X seconds to get the updated price. Using websockets, the SERVER could directly send the new price to the web browser as soon as it has changed.

You may also be interested in looking into long polling which is a techinc used with HTTP GET / POST to provide similar functionality as Websockets (though it is a totally different thing)