Entities is a form of concept describes a collection of instances of interest. An entity typically has attributes.
Relationship types typically have cardinality constraints, dictating the number of instances of the respective entity sets that are related by a relationship of that type. UML also allows arrowheads as annotations on associations. These arrowheads play no role in the relationships themselves, but are UML notation for specifying navigability.
XML is an open family of languages in the sense that developers define their own application-specific dialects. DTDs do not support namespaces, which remains a serious deficiency.
