web service in SharePoint architecture

Hi all!

I'm trying to understand the term "Web service". I found many articles, but please tell me in simple words -

1.What is the web service?What is it used for?

2.One web service can contain one or more web applications?

3.http://msdn.microsoft.com/en-us/library/office/ee705814(v=office.14).aspx  In SharePoint 2010 there are many web services, but what service  SPWebApplication.WebService return? 

thanks