Suitelets are extensions of SuiteScript that allow developers to write custom NetSuite pages and backend logic. Suitelets are server side scripts that operate in a request-response model. They are invoked by HTTP GET or POST requests to system generated URLs, and by default they are invoked by making a GET request from a browser.
Before beginning with SuiteScript (suitelet) development, you must enable the Server SuiteScript feature to use it and also enable Netsuite to display "Internal ID" of entities in the preferences.