XPages base internally on JavaServer Faces, but XPages developers don't have to have JavaServer Faces skills. The sources of a XPages pages are an XML derivat called XSP which is translated into Java code when running the applications. At runtime this code returns the HTML to various clients.
Tooling with Domino Designer
XPages applications are stored in NSF (Notes Storage Facility) files which are collections of design elements that can be deployed as a whole and developed using IBM Lotus Domino Designer.
In order to access data XPages and Domino Designer use so called data sources. These data sources can be bound graphically to UI controls on pages. UI controls and other types of controls show up in a palette of controls in Domino Designer and can be dragged and dropped onto pages. Domino Designer also comes with futher convenience functionality, for example field validations, translations, etc.
XPages functionality can be modularized in so called 'Custom Controls' which are XPages controls that can be reused in multiple XPages pages and parameterized based on context.
XPages Runtime Extensibility
The XPages runtime can be extended using the XPages Extensibility API which is based on JavaServer Faces. This capability can for example be used by ISVs who want to provide their own libraries of XPages controls.