To a large degree much of the discussion of entities is more relevant in
the next section, writing "valid" documents, rather than in this section,
writing "well-formed" documents.
As such, we will discuss entities in greater details in the next section.
Nevertheless, some issues make sense within this section, because entities
must be well-formed as well as valid. So, in this section, we will
introduce entities in terms of their basic syntax and leave the nitty
gritty for a little bit later.
As we said before, entities are essentially aliases that allow you to
refer to large sections of text without having to type them out every time
you want to use them.
Suppose you have your letterhead saved as an entity in a shared file.
Then, every time you write a letter in XML, you might say something like
<LETTER>
&letterhead;
<TO>Bob Frog</TO>
<BODY>
blah blah blah
</BODY>
<FROM>Kevin Kelly</FROM>
</LETTER>
Notice that the letterhead might expand out to
My Company
1234 Fifth Ave.
Suite 1256
Los Angeles, California 90026
However, instead of typing that out in every letter, you just use
&letterhead;
There are two types of entities, general and parameter entities and each entity has two parts,
the declaration and the entity reference.
