XLinks can do everything HTML links can do and quite a bit more, but they aren't supported by current applications.
XLink elements of all types are placed in the
http://www.w3.org/1999/xlink namespace, normally with
the xlink prefix.
Simple links behave much like HTML links, but they are not
restricted to a single <A> tag.
Linking elements are identified by xlink:type
attributes.
Simple link elements are identified by
xlink:type attributes with the value simple.
Linking elements can describe the resource they're linking to
with xlink:title and xlink:role attributes.
Linking elements can use the xlink:show attribute to
tell the application how the content should be displayed when
the link is activated, for example, by opening a new window.
Linking elements can use the xlink:actuate attribute to
tell the application whether the link should be traversed
without a specific user request.
Extended link elements are identified by
xlink:type attributes with the value extended.
Extended links can contain multiple locators, resources, and arcs. Currently, it's left to the application to decide how to choose between different alternatives.
A resource element represents a local, inline
resource. It is identified by an xlink:type
attributes with the value resource.
A locator element represents a remote, out-of-line resource.
It is identified by an xlink:type
attribute with the value locator.
Both locator and resource elements can be labeled by
xlink:label attributes. These labels are
used to define arcs between resources.
A locator element has an
xlink:href attribute whose value is the URI of the
resource it locates.
Arc elements are identified by xlink:type
attributes with the value arc.
Arc elements have xlink:from and xlink:to
attributes of IDREF type that identify the
resources they connect by their labels.
Arc elements may have xlink:show and
xlink:actuate attributes to determine when and how
traversal of the link occurs.
An out-of-line link is a link that does not contain any local resources.
A linkbase is a document containing multiple out-of-line, extended link elements.
A linkbase is found when a document with an extended link with the role xlink:external-linkset is read.