The document structure is called the document tree . The main trunk of the tree is the parent. All the branches and leaves are children. Document trees are usually visually represented as a hierarchal chart . The most important thing to remember about a structured document is that it is defined by the elements it contains, not by how it looks.
Structure says that an element is a paragraph.
Format says to display the paragraph in 12 point Times. Structure says the element is a book title. Format says to display the book title in green bold body text. Structure say the element is a social security number. Format says to hide and not display the social security number.