The first thing will be to understand what a web application is and how it differs from a “common” application.
The applications we have decided to call “common” are those that are installed and used on a computer. Applications designed to work on an operating system (Windows, iOS, Linux, …), it is usual to call them desktop applications, because they are applications designed to be used on a desktop computer, but also on a laptop. It is important that we do not confuse “common” with “simple”: applications can have enormous levels of complexity whether they are web or desktop applications, they are two elements that are not related.
The difference of web applications is that they are designed to work on a server on the Internet, which you access with your browser. They are usually applications that require you to create an account and can use it using a username and password.
At present, with the level of connectivity that exists on a global scale, it is common that whenever a company needs a software solution, think first if it is more convenient to do so with a web application. When analyzing the advantages and disadvantages of having a web application instead of a desktop application, the balance clearly leans towards the web application. [EVERYTHING: make a post talking about this comparison between web applications and desktop applications]
In a web application there are no compatibility problems with operating systems, you only need a browser, and although not all browsers are good [EVERYTHING: Make a post about the different browsers and which we recommend] surely with the one you use you can always exploit 100 % any tool developed in the form of a web application.
Very good very good, but … and then what is a web service?
A web service is something a bit more complex to explain. Not all companies have business models that require at some point to develop a web service, it is usually related to companies that offer information services on the Internet. A clear example would be a company dedicated to weather forecasts. A web service that this company could offer is to be able to deliver its forecasts to its clients completely autonomously through an API (the issue is already getting complicated, but we will try to keep it as simple as possible). This would be a way in which other programs can communicate with the forecasts offered by the company and can obtain forecasts automatically. Thus, customers who use these forecasts could have their own business app that automatically download data from the latest forecasts and serve as a powerful working tool.
Trying to simplify to the maximum, the difference between a web application and a web service is that the web application is a management system designed to be used by humans, while a web service is a system designed to be used by other programs.