It can be thought of as a tree of elements--containers of information.

Traditionally, from a markup perspective:

• Articles, books, notes, poems, novels

• Technical manuals, slip sheets, product packaging

More abstractly, documents are instances of information that typically have structure.

So, lots of things are documents:

• Messaging, e-mails, web sites, etc.

• Business transactions, invoices, statements, etc.

• Log files, configuration files, install scripts, etc.

• Ontologies, medical lab data, instrument experiment results, human genome annotations, etc.

XML is a useful common syntax for encoding these documents.