XML is a markup language with some general but strict rules. Its
actual structure varies widely. iTunes playlists, MS Word document
metadata, and yes xhtml. not old html, but a form of it that is
replacing it and is compliant with xml rules. (more or less)
Most xml files are probably hashes with nested hashes (associative
arrays with nested associative arrays), and they follow the KVC
format, Key=>Value coding. Great for storing data and preferences and
all kinds of things. And easy to change from one format to another.
XML is a pretty simple, but often confusing topic. Most books treat
it as a subject distinct from anything else, but it isn’t.
There are some Java and XML books, for example that introduce XML in
a working environment.
Just remember it’s structured data, stored as a text file.
Conceptually, not different from YAML, just more strict.