A Web Service is a processing point that is called on an online address, receive an input request and produce an output response.
The request and response are data structures previously defined of a certain Service Data Type like Binary, Text, Json, File, etc. and the type of processing is defined by one of the Web Service assigned to a Service Data Type.
The Web services can have implementation for multiple languages and versions like PHP 7.1.26, Python 3.7, Java 1.8, as a consequence there will be only one entry point for each language and version, and then you specify inside the request which web service is called.
The web services are implemented like functions, having an unique name, for which is specified input request data type that they can process, and output response data type that they can produce. The function implementing the web service will receive 2 parameters input and output as array/dictionary/map (the list of variables accessed by name most common to each language), and can also access external web services or internal web service methods in cascade if accessible.
The web services could be accessed using Rest service, AJAX or SOAP protocols, or you can schedule a web service call later or periodically.
To have access to a web service you have to buy the access for a period, or for an amount of data processed, even if some of them can be free. For demo purpose you can have access for a number of calls.
You can also execute the web services for which you have access, on your computer, by downloading a client for this website.