Firstly,What do we mean by Web services?

Web Services are client and server applications that communicate over the World Wide Web’s (WWW) Hypertext Transfer Protocol (HTTP). In other words its simply a ‘method’ or ‘function’ in a normal programming language, except you are calling it over the internet.

For example:
Lets say you are creating a website that shows weather information of cities. You can actually get the data for the cities for already exposed web services. You will get the response in form of XML or JSON, you can parse it and show it on your website.

Okay now what meant by RESTful web services?

A RESTful web application shows information about itself in the form of
resource information. It also allows the customer to take action on these
resources such as creating new resources (example- creating a new user) 
or changing existing resources through HTTP verbs

Resources – They describe the “things” that a client may want to pull from a server 
(e.g. documents and files that make up a web page).

HTTP verbs- Which are the web verbs — they describe the “actions” you can take on a resource, such as GET, POST, PUT, and DELETE.

Application program interface (API) – HTTP requests to GET, PUT, POST and DELETE data.

Now Lets Discuss what is express JS?

It is a web framework that allows you to structure a web application in a specific url to handle multiple different http requests. 

Express is a flexible framework for Node.js web apps designed to make it much easier to develop websites, web apps, and APIs.

Why use expressJS ?

Basically, You can use few lines of codes with express JS to do a complex task with ease and also supports multiple templating engines to simplify generating HTML.

example: Express helps you with route support to respond to requests so you can write responses to specific URLs

How to Install & use Express ?

You can simply get it through NPM as follows:

>>npm install express