A container does not initialize the servlets as soon as it starts up, it initializes a servlet when it receives a request for that servlet first time. This is called lazy loading. The servlet specification defines the element, which can be specified in the deployment descriptor to make the servlet container load and initialize the servlet as soon as it starts up. The process of loading a servlet before any request comes in is called preloading or preinitializing a servlet.
tag in the deployment descriptor web.xml preinitialization of a servlet can be done.
Servlet are loaded in the order of number(non-zero-integer) specified. That is, lower(example: 1) the load-on-startup value is loaded first and then servlet with higher values are loaded.