With the basic parts of XML markup defined, there is one more component we need to look at. An entity is a placeholder for content, which you declare once and can use many times almost anywhere in the document. It doesn't add anything semantically to the markup. Rather, it's a convenience to make XML easier to write, maintain, and read.
Entities can be used for different reasons, but they always eliminate an inconvenience. They do everything from standing in for impossible-to-type characters to marking the place where a file should be imported. You can define entities of your own to stand in for recurring text such as a company name or legal boilerplate. Entities can hold a single character, a string of text, or even a chunk of XML markup. Without entities, XML would be much less useful.
You could, for example, define an entity w3url to represent the W3C's URL. Whenever you enter the entity in a document, it will be replaced with the text http://www.w3.org/.
Figure 2.13 shows the different kinds of entities and their roles. The two major entity types are parameter entities and general entities. Parameter entities are used only in DTDs, so we'll describe them in Chapter 5. In this section, we'll focus on the other type, general entities. General entities are placeholders for any content that occurs at the level of or inside the root element of an XML document.
An entity consists of a name and a value. ...