The Java Servlet API allows a software developer to add 
dynamic content to a Web server using the Java platform. 
The generated content is commonly HTML, but may be other 
data such as XML. Servlets are the Java counterpart to non-
Java dynamic Web content technologies such as PHP, CGI and 
ASP.NET. Servlets can maintain state across many server 
transactions by using HTTP cookies, session variables or 
URL rewriting.

The Servlet API, contained in the Java package hierarchy 
javax.servlet, defines the expected interactions of a Web 
container and a servlet. A Web container is essentially the 
component of a Web server that interacts with the servlets. 
The Web container is responsible for managing the lifecycle 
of servlets, mapping a URL to a particular servlet and 
ensuring that the URL requester has the correct access 
rights.

A Servlet is an object that receives a request and 
generates a response based on that request

Servlets exist in and are managed by the servlet engine in
the Application Server. The servlet engine is an internal
object that handles all servlet meta functions. These
functions include instantiation, initialization,
destruction, access from other components, and configuration
management