Servlets are the core of web-application development. A servlet is a Java program that runs on the server side, either on a web server or an application server. Servlets acts as a middle layer between an HTTP server and a web client, typically a web browser. The client can either be an HTTP client or a service that runs on an HTTP server. The servlet technology is the underlying technology behind JSP, which is used to create UI pages using HTML. JSP is compiled to a servlet. When a request is made by a web client, the servlet container loads the servlet. The servlet accepts the request, processes it, and returns the response to the servlet container. The servlet container in turn sends the response to the web client that issued the request. The servlet remains in memory to serve subsequent requests and the servlet gets removed from the container when it is no longer needed or the servlet container restarts.