The XML Linking Language (XLink), which allows elements to be
inserted into XML documents in order to create and describe links between
resources.
XLink offers two kinds of links:
Extended links :
An extended link is a link that associates an arbitrary number of resources. The XLink element type for extended links is any element with an attribute in the XLink namespace called type with a value of "extended". An extended link indicates its participating local resources by means of special subelements that appear inside the extended link.The XLink element for local resources is any element with an attribute in the XLink namespace called type with a value of "resource". An extended link indicates remote resources that participate in it by means of locator elements. The XLink element for locators is any element with an attribute in the XLink namespace called type with a value of "locator".An extended link may indicate rules for traversing among its participating resources by means of a series of optional arc elements.The XLink element for arcs is any element with an attribute in the XLink namespace called type with a value of "arc".
Simple links :
Simple links offer shorthand syntax for a common kind of link, an outbound link with exactly two participating resources. A single simple linking element combines the basic functions of an extended-type element, a locator-type element, an arc-type element, and a resource-type element. The XLink element for simple links is any element with an attribute in the XLink namespace called type with a value of "simple".
XLink Element Type Attribute (type)
The value of the type attribute must be supplied. The value must be
one of "simple", "extended", "locator", "arc", "resource", "title", or
"none".
Locator Attribute (href)
The attribute that supplies the data that allows an XLink application
to find a remote resource (or resource fragment) is href. It may be used
on simple-type elements, and must be used on locator-type elements.
Behavior Attributes (show and actuate)
When used on a simple-type element, they signal behavior intentions
for traversal to that link's single remote ending resource. When
they are used on an arc-type element, they signal behavior intentions for
traversal to whatever ending resources (local or remote) are specified
by that arc.
The show attribute is used to communicate the desired presentation
of the ending resource on traversal from the starting resource.
The actuate attribute is used to communicate the desired timing of
traversal from the starting resource to the ending resource.
foe details refer w3c page