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XML lets you make up names for your own element types. If you think tags and elements are the same thing you are already in considerable trouble: read the rest of this question carefully.
The same applies if you are thinking in terms of ‘fields’ (see How do I get XML into or out of my database?). Wrong paradigm, wrong language.
Bob DuCharme writes:
Don't confuse the term ‘tag’ with the term ‘element’. They are not interchangeable. An element usually contains two different kinds of tag: a start-tag and an end-tag, with text or more markup between them.
XML lets you decide which elements you want in your document and then indicate your element boundaries using the appropriate start- and end-tags for those elements. Each <!ELEMENT... declaration defines a type of element that may be used in a document conforming to that DTD. We call this type of element an ‘element type’. Just as the HTML DTD includes the H1 and P element types, your document can have color or price element types, or anything else you want.
Normal (non-empty) elements are made up of a start-tag, the element's content, and an end-tag. <color>red</color> is a complete instance of the color element. <color> is only the start-tag of the element, showing where it begins; it is not the element itself.
Empty elements are a special case that may be represented either as a pair of start- and end-tags with nothing between them (eg <price retail="123"></price>) or as a single empty element start-tag that has a closing slash to tell the parser ‘don't go looking for an end-tag to match this’ (eg <price retail="123"/>).