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Before we get started further explaining web services and APIs, we first need to define a few other terms:

• XML [eXtensible Markup Language] is a standardized format for storing and sending data. Similar to HTML, XML stores data by wrapping it in descriptive tags.
• JSON [JavaScript Object Notation] is similar to XML in that it also stores and enables you to send data in a standardized format. JSON just uses a different, object-based, methodology for systematically storing data.
• HTTP [HyperText Transfer Protocol] is the foundation of transferring data and communications on the internet.
• SOAP [Simple Object Access Protocol] is a messaging protocol used for exchanging structured information[XML data] over a network.
• REST [REpresentational State Transfer] is a standardized architectural style that can be used when creating a web API.
• Web applications (Web app) are computer programs that are accessed over the internet through a computer’s web browser.

A Web service is a way for two machines to communicate with each other over a network. A webserver running on a computer listens for requests from other computers. When a request from another computer is received, over a network, the Web service returns the requested resources. This resource could be JSON, XML, an HTML file, Images, Audio Files, etc.

The architecture of Web service:

An API, or Application Programming Interface, is a set of definitions and protocols that allow one application to communicate with another application. In general, when we speak about APIs, we are likely speaking about web APIs [APIs that are accessible over the internet]. This is not always the case though. APIs can be exposed through local files (such as a JAR file in a Java program, .H file in C/C++ programs, etc.) to allow two local applications to communicate with each other. This doesn’t require a network as the two applications are communicating within a single device.

The architecture of API:

As said earlier there are lots of difference between web service and API “All Web Services are APIs, but not all APIs are Web services”.

Is a REST API a Web Service?

Let’s say that you want to see what your best friend posted on Instagram. To do this, you need to go on the app and open up your friend's Instagram page. In this example, your Instagram app [the client], would make a request to Instagram’s server [the server] to request your friend’s Instagram profile. This request would be a GET request to the /users endpoint and in the parameters of the request your friend’s account ID would be included.

• HTTP Method: GET
• URL: https://api.instagram.com/v1/users/
• Parameters: user={best_friends_user_id}

In the same way that you use a GET request to retrieve data, a POST request would be used to create data on a platform. So let’s use the example of posting an image to Instagram. This request would be a POST request to the /media endpoint with a body of the image and parameters with your caption.

• HTTP Method: POST
• URL: https://api.instagram.com/v1/media/
• Parameters: caption={my_great_caption}&user={my_user_id}
• Body: Image to upload

The reason REST is so great is that it offers a standardized methodology for making requests to an API. Once you learn one REST API, other REST APIs are going to function in a similar way.

If an API is available over the internet, there is no need to install additional software within your application. You can access the data from any application that is connected to the same network as the API.

With separated development on the client and server, the client code can be updated without affecting the server, and the server code can be updated without affecting the server. This is assuming the changes are developed in a backward-compatible way.

Advantages of API Services

• API supports traditional CRUD (Create Read Update Delete) actions as it works with HTTP verbs GET, PUT, POST, and DELETE.
• API helps you to expose service data to the browser
• It is based on HTTP, which is easy to define, expose in REST-full way.

Advantages of Web Services

• Offers faster communications within and across organizations
• Each service exists independently of other services.
• Interoperability has the highest priority.
• Using Web services, your application helps you to publish its message or function to the rest of the world.
• Web services help solve interoperability issues by giving different applications a way to link their data.
• Web services help you to exchange data between different applications and different platforms.
• It allows applications to communicate, exchange data, and shared services among themselves.