Do you understand the idea behind client-server?
That is basic to understanding ANY server-side scripting or application language. JSP and Java Servlets are very much the SERVER side of application programming.
Tomcat is the server that will run your Java Servlets and JSP scripts. Other Java servers are Web Logic and WebSphere. These are very much like IIS running .NET and ASP applications on Windows.
For the web, if you don't have a server to connect to, you will never have an application that other clients can access.
Einstein47
("Tribulations are God's megaphone to awaken a deaf world." - C.S. Lewis)