A Servlet is a Java application programming interface (API) running on the server machine which can intercept requests made by the client and can generate/send a response accordingly. A well-known example is the HttpServlet which provides methods to hook on HTTP requests using the popular HTTP methods such as GET and POST. You can configure HttpServlets to listen on a certain HTTP URL pattern, which is configurable in web.xml, or more recently with Java EE 6, with @WebServlet annotation. Many Java EE web frameworks are built on top of servlets, such as JSF, JAX-RS, Spring MVC, Struts, Wicket, etcetera. See also http://stackoverflow.com/questions/2095397/what-is-the-difference-between-jsf-servlet-and-jsp/