XPages is a web Framework for IBM Notes platform. It was introduced in Lotus Domino 8.5 (has to be verified).

It is based on JSF (JavaServer Faces) with a lot of useful extensions to represent and manipulate IBM Notes data.

IBM offers a tutorial for XPages: https://www-10.lotus.com/ldd/ddwiki.nsf/dx/Tutorial-intro-to-XPages.htm

Behind the Scenes In backend XPages are XML files which is similar to HTML. The Domino Server finally makes real HTML and send these pages to the Client. The Business logic is implemented in native JavaScript ("clientside JavaScript - CSJS") and pseudo JavaScript ("serverside JavaScript - SSJS").