An XML document that uses namespaces has one or more namespace declarations, which appear as attributes of elements. A namespace declaration might appear as an attribute of any element. Frequently, the document element (the one that encloses all the other elements) has a global namespace declaration or declarations.

A namespace declaration associates a prefix with a URI. Once associated, the prefix might appear with element names, attributes names, or both, to distinguish the names from identical names that might mean something else. For example, an XML element whose name is memory might mean completely different things and have different valid attributes, depending on whether it appears in a computer inventory or in a psychological report. You can distinguish the uses by having a computer-inventory namespace and a psychological-report namespace.

In the SAX2 interface, XML namespaces affect the values of the parameters of the StartElement and EndElement callback procedures, as well as the attribute data in the attributes parameter of the StartElement callback. There can be slight variations in the way that the SAX-reader object handles namespace processing, depending on the configuration of the SAX-reader object.