You might have read (about) the book Domain-Specific Languages written by Martin Fowler. As the name suggests it is about these little useful programming languages you can built very easily with Xtext.
Papyrus is aiming at providing an integrated and user-consumable environment for editing any kind of EMF model and particularly supporting UML and related modeling languages such as SysML and MARTE. Papyrus provides diagram editors for EMF-based modeling languages amongst them UML 2 and SysML and the glue required for integrating these editors (GMF-based or not) with other MBD and MDSD tools.
Papyrus also offers a very advanced support of UML profiles that enables users to define editors for DSLs based on the UML 2 standard. The main feature of Papyrus regarding this latter point is a set of very powerful customization mechanisms which can be leveraged to create user-defined Papyrus perspectives and give it the same look and feel as a "pure" DSL editor.