Loading and starting. .. well the general term used is deploying an
application. It will take too  many lines to post here, to explain what
happens. In short, you may understand it this way:

1. To deploy an application in Tomcat, you create a folder inside the
webapps folder in the tomcat install dir. That folder is now the
context root of your application.
eg. c:\tomcat\webapps\myApp
2. inside myApp, there should be a folder called WEB-INF, which should
have web.xml file. (deployment descriptor). And the class files should
be present as per the package hierarchy inside WEB-INF/classes.

Search google for deployment descriptor and deploying in tomcat. Also
search for web application structure in tomcat.