Hyper text differs from the normal text in that they have hyperlinks. The hyperlinks are identified by the characteristic blue color underlined text that identifies hot spots. These hotspots when clicked, will take us to the Web pages that are specified as links.

Linking or cross-referencing can be done when :

	There is a need to provide context-sensitive help, for instance, when we navigate tutorials, it will be easy to understand if elaborations for some technical terms or external references are provided.
	A file has to be referred or displayed on clicking the mouse at a particular point of the document.

Link is a functionality that is associated with a text or an object in a document using markup language.

Providing links enable us to refer to an object or a file within the same document or a document in some other location. The object which is referred to, can be an image file or any other file, or text within the same document, or a different document. Within a HTML or an XML document, there may be several links. The active document, which contains these links, is called the source file.

The linked file or file that has been referred to is called the destination file or the target.

The target file may be directly opened on clicking a hotspot or it may in turn lead to some other link. In simple terms of HTML linking, a link is the association between a source and the target.
The target may be completely a new HTML page, in which case, the description of the target (locator) would be a Universal Resource Identifier (URI). The target may be a named element within an HTML page. It can be identified using the # symbol, called a fragment identifier. Then, the fragment identifier has to be followed by the NAME attribute of the target element.

Both the source and the target file can be viewed simultaneously. One of the best examples of viewing both the source and the target simultaneously, is the search index provided in the HELP menu of any application, wherein selecting a particular index opens the corresponding document.

The differences between HTML links and XML links are depicted in the table shown here.

There are six different areas that need to be addressed while dealing with linking and querying. They are as follows.

XML information set - The World Wide Web Consortium (W3C)' s document defines what an XML information set is. It says that XML information set comprises various pieces of information, which together make up the XML document.

Xlink is W3C' s mechanism for linking to other resources within an XML document. Xlink also allows no-XML document to be linked together. XPath is the general language specification for addressing parts of an XML document framed by W3C' s.

XPointer is W3C' s mechanism for pointing to a particular location within an XML document.

XML fragment Interchange for transmitting a part of XML document as per W3C specification.

Querying XML document -XML document can be queried using XSLT technology recommended by W3C, which is dealt with in the later part of the session.

The XML information set, or Infoset is a working draft created by W3C to describe various pieces of information that together form an XML document. There are fifteen distinct types of information that forms the XML document. They are represented in the table as shown.

	Exactly one document information item.
	One or more element information items.
	Attribute information items.
	Processing instruction information items.
	Character information items.
	Reference to skipped entity information items.
	Comment information items.
	A document type declaration information items.
	Entity information items.
	Notation information items.
	Entity start marker information items.
	Entity end marker information items.
	CDATA start marker information items.
	CDATA end marker information items.
	Namespace declaration items.

HTML has two link elements namely, A and IMG, whereas in XML links, the link elements are identified by the element attributes.

Any XML element can act as a link element provided it has the right kind of attributes.

<|ELEMENT CORRELATION ANY> <!ATTLIST CORRELATION xlink:form CDATA #FIXED value>

The primary attribute that identifies the XML element as a link is the xlink:form attribute, whose declaration in an XML DTD, would be as shown . Here the value should be a locator and not the linking elements. Also the value can be simple or extended.

XML links work with link elements. The link elements in turn contain locators.

Locators are in the form of attributes or other elements that points to specific locations.
In general, a locator is a URI, a fragment identifier, or a URI combined with a fragment identifier. Locators for XML documents are extended pointers.

The syntax of locators allows us to use the two variations as shown here.
URI#fragment - This fetches the whole of the resource identified by the URI and then extracts the part identified by the fragment identifier.

URI|fragment - The application can decide how it will process the URI in order to extract the resource. This could be used to retrieve a particular part of the document.

If the fragment identifier is a character string, the string is treated as the value of the id attribute of an XML element. For instance, the locator sample.html#sa2 would point to the element with attribute value of sa2 in the file sample.html.

XML' s Xlinks are used to establish hyperlinks in XML documents.

The W3C Xlink working draft defines two categories of links. They are, simple links and extended links.

The xml:link attribute is used for specifying a link or location term as shown here.

xml:link="simple"| "extended" | "locator" | "group" | "document"

The figure here shows the outline classification of XML links.

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