ASP.NET is a server-side web application framework released by Microsoft in 2002 that includes Web Forms, MVC, Web Pages, and Web API. ASP.NET AJAX uses extensions like the ScriptManager, UpdatePanel, Timer, and UpdateProgress controls to enable partial page updates with AJAX functionality in ASP.NET applications without full page refreshes. The core ASP.NET AJAX controls allow automatically refreshing parts of ASP.NET pages at regular intervals or providing progress status for partial page updates.