All XML documents may be understood in terms of the data they contain and the markup that describes that data. Data is typically "character data" (letters, numbers, punctuation...anything within the boundaries of valid Unicode) but can also be binary data. Markup includes tags, comments, processing instructions, DTDs, references, etc....

The most simple example of character data and markup would be something like the following:

<NAME>Selena Sol</NAME>

In this case, the <NAME> and </NAME> tags comprise the markup and the "Selena Sol" comprises the character data. As you can imagine there are few rules that manage your data (content) other than what type of data is allowed (binary or ascii for example). On the other hand, there are many rules that define how you must code your markup.

In the rest of this section, we will outline what requirements you must satisfy in order to write well-formed XML.