Well this point should be learned first when you are going to write your first servlet. A web server understand a web application and its element by its web.xml. For your info following is a brief explanation.
<Servlet-lass> has been assigned a new name <servlet-name> in the <servlet> tag and for that particular name is mapped in <servlet-mapping> tg to a <url-pattern> so all the url ending with that specific url will be handled by the mapped servlet. that's all.
For more info try google, search your book this explanation must be mentioed somewhere.