Web Services is the mechanism or the medium of communication through which two applications / machines will exchange the data irrespective of their underline architecture and the technology.
In the modern era of technology if you want to build a software application you don’t need to build each and everything from scratch. There are lots of readymade services available which you can plug into your application and you can start providing those services in your application.
For example you want to display weather forecast information you don’t need to collect, process and render the data in your application. You can buy the services from the people who already well-established in processing and publishing such kind of data.
Web services allow us to do these kind of implementations.
Tony Gurney 