Servlet Architecture

Servlets read the explicit data sent by the clients (browsers). This includes an HTML form on a Web page or it could also come from an applet or a custom HTTP client program.
Read the implicit HTTP request data sent by the clients (browsers). This includes cookies, media types and compression schemes the browser understands, and so forth.
Process the data and generate the results. This process may require talking to a database, executing an RMI or CORBA call, invoking a Web service, or computing the response directly.
Send the explicit data (i.e., the document) to the clients (browsers). This document can be sent in a variety of formats, including text (HTML or XML), binary (GIF images), Excel, etc.
Send the implicit HTTP response to the clients (browsers). This includes telling the browsers or other clients what type of document is being returned (e.g., HTML), setting cookies and caching parameters, and other such tasks.

Servelt API contains three packages
javax.servlet: Package contains a number of classes and interfaces that describe the contract
between a servlet class and the runtime environment provided for an instance of such a class a
conforming servelt container.
javax.servlet.aanotation: Package contains a number of annotations that allow users to use
annotations to declare servlets , filters, listeners and specify the metadata for the declared component
javax.servlet.http: Package contains a number of classes and interfaces that describe and define the contract between a servlet class rnning under the HTTP protocal and the runtime environment provided for an instance of such class by a confirming servlet container.