What is XML and what is its basic structure?

XML (Extensible Markup Language) is a text format that encodes structured data as a tree of nested, freely-named tags.

Every piece of data sits inside an element, written with a matching start- and end-tag: <name>Anna</name>. Elements nest inside other elements, so the whole document forms a tree with one root element at the top and the actual values in the leaves:

<offer>
  <store>
    <name>Amazon</name>
    <url>http://www.amazon.com</url>
  </store>
  <book>
    <isbn>1575213966</isbn>
    <price>
      <amount>23.99</amount>
      <currency>USD</currency>
    </price>
  </book>
</offer>

The "Extensible" part is the key idea: the tag names aren't fixed by a standard — you invent <offer>, <book>, <isbn> to describe your own data. That flexibility is why XML can model almost any structure.

Tip: Read XML as a tree, not as text. <offer> is the trunk; <store> and <book> are branches; Amazon and 23.99 are the leaves you actually want.

Go deeper:

• XML (Wikipedia) — origins, structure, and how XML compares to other data formats.