Web services are a type of internet software that use standardized messaging protocols and are made available from an application service provider's web server for web users and other web-connected programs. They do not provide the user with any graphical interface; instead, they share business logic, data, and processes through a programmatic interface across a network.
Developers can then add the web service to a GUI (such as a web page or an executable program) to offer specific functionality to users. They allow different applications from different sources to communicate with each other without time-consuming custom coding.
They can be used with virtually any programming language that supports HTTP and XML, which makes them highly versatile.