Otherwise, a browser that reads the XML won't know the difference between one picture and another. Each picture in Listing A is a second level element. This drilling down is called nesting . The information contained within the tags on each level is the data you're making available to the end user. In addition to storing data, elements can contain attributes.
Attributes help describe elements and what they contain.
You can add as many attributes as you want as long as you separate them with spaces. It's helpful to think of elements as nouns and attributes as adjectives. The example in Listing A can be changed so that it uses attributes extensively. In this example, it makes sense to use attributes instead of nested tags if you wish to treat the name, copyright information, and file format as attributes of an image file instead of as data. Conforming to XML standards Files that conform to the XML standard are known as being well-formed.