This article describes  the overall servlet architecture and what you need to develop your application with servlets.

Sun Microsystems  tutorial on  servlets

Sun Microsystems brief tutorial on how to write servlets.

This document explains the concepts of Java Servlets and provides a step-by-step tutorial for writing HTTP Servlets with complete source code for the example Servlets. The tutorial and the other chapters cover all facets of Servlet programming from a simple "Hello World" Servlet to advanced Servlet features like session tracking

This tutorial will attempt to explain how to write Java servlets and use them with a real webserver, such as Apache.

Java Tips - Introduction to Java Servlets with Eclipse