Sorry Jezelll, had to throw a downvote. This is not what I would consider 'plain english' :( I don't have a problem with the intent of your definition, but using words like 'operation', 'remote server', 'invoked' and 'specification' makes it hard for beginners to understand. See Mitchel Sellers answer as an example of 'plain english'. – Dhaust Jun 30 '09 at 23:53

A web service is any piece of software that makes itself available over the internet and uses a standardized XML messaging system. XML is used to encode all communications to a web service. For example, a client invokes a web service by sending an XML message, then waits for a corresponding XML response. As all communication is in XML, web services are not tied to any one operating system or programming language—Java can talk with Perl; Windows applications can talk with Unix applications.
I’d stay away from imgur. They’re basically a social media site now, with reputations that go up or down, presumably based on other users’ opinions of you. They’ve also changed the way their albums work-now if you make an album public, you can’t add any new images, and I could find no instructions on how to UNDO making an album public. Their organization style is awful, making it almost impossible to find anything you might actually want. The front page features their “hot” images, which are a mixed bag of political BS, things you might want to see, and vulgar/gross imagery.

A web service is a collection of open protocols and standards used for exchanging data between applications or systems. Software applications written in various programming languages and running on various platforms can use web services to exchange data over computer networks like the Internet in a manner similar to inter-process communication on a single computer. This interoperability (e.g., between Java and Python, or Windows and Linux applications) is due to the use of open standards (XML, SOAP, HTTP).

Think of the name you want to register. The answer is typically your company or website name. It is best to keep your domain name short and easy to understand. Say it out loud, and make sure it sounds great. Next, search to see if it is available. If the name you desire is taken with the .com top-level domain, there are hundreds of others available. Finally, add the top choices to your cart and complete the domain registration.
Unlike traditional client/server models, such as a Web server/Web page system, Web services do not provide the user with a GUI. Web services instead share business logic, data and processes through a programmatic interface across a network. The applications interface, not the users. 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.