Servlets are miniature programs that must be run inside another program called a container. The container is responsible for brokering communication between the servlet and the outside world. Beyond that, servlets are like every Java application. They can import packages, write to the hard drive, and even install viruses on your machine. By default, servlets are trusted code. The reason is that if someone has enough permission on your computer to install a servlet, he already has enough power to tie the whole machine in knots anyway. The thought is that this installer is a trusted person, so the work that he does should be trusted, too.
Servlets are not Java applications. They must be run inside a container instead of from a command line. That being said, you can add almost any functionality available to a Java application to a servlet also.
Servlets are not the only way for the server to communicate with a browser. A Web server is an HTTP application. It receives communication via the HTTP protocol and it communicates with the browser via HTTP also. You can write your own HTTP server-side application that processes its own requests, bypassing both servlets and the servlet container. This chapter contains a simple example of how that is done.
