Java Servlet
A Servlet is a Java programming language class used to extend the capabilities of a server. Although servlets can respond to any types of requests, they are commonly used to extend the applications hosted by web servers, so they can be thought of as Java Applets that run on servers instead of in web browsers. These kinds of servlets are the Java counterpart to non-Java dynamic Web content technologies such as PHP and ASP.NET.
Read more about Java Servlet: Introduction, History, Advantages Over CGI, Usage