Traditionally, the standard way to perform server-side processing for a web application has been to write a CGI program. Servlets are a Java-specific replacement for CGI. A servlet is a Java class that implements the Servlet interface. Typically, but not necessarily, the servlet will service HTTP requests. The Servlet API provides assistance in dealing with HTTP requests and responses, unlike CGI where you have to program every detail yourself. CGI programs have a high overhead, since a new process has to be created to service each request. Servlets can be much more scaleable, since the Web server can keep a pool of threads and execute the servlets in these threads.
The book starts with an (optional) introduction to web development. The next section covers the fundamentals of servlets: the lifecycle, API calls and the basics of running and debugging servlets. It then moves on to more advanced topics, such as thread safety, cookies, state management and database access. After this it presents complete source code for a few sample servlets and finishes with a convenient quick reference to the servlet API. The book comes with a CDROM which contains not only the usual source code and JDKs, but also trial versions of two servlet engines - particularly useful if your web server does not support servlets (and if it's a Microsoft one it won't).
The book assumes basic Java knowledge, but little else. It gives a good coverage of the basics: the sections on configuring servlet engines and debugging servlets are particularly useful. However the coverage of advanced material is rather sketchy: for example the JavaServer architecture gets only 10 pages and Java Server Pages are not mentioned at all. If you only need the basics, this is a good place to start. If you really need advanced coverage, you will have to look elsewhere.