Entities: What are They Good For?
Q: Tell me about entities in XML documents. What are they and how do I use them?
A: After you're comfortable with XML markup, it's time to tackle entities. The term entity in the XML Recommendation is used for several related, but slightly different things. There are three things that might loosely be called entities in XML, and we'll take a detailed look at each of them:
Internal entities function as typing shortcuts or macros.
External entities allow you to incorporate content from other files.
Parameter entities, which can be internal or external, are only available within the internal and external subsets (the DTD).