XPath in a nutshell

XPath expressions are used to navigate in the XML document. XSLT uses XPath

.Net supports XPath queries through classes like XPathDocument, XPathNavigator and XPathIterator.

XPath expressions and their meaning.
Following are the expressions which can be used for entire document search. This is an absolute search not relevant to the current node in the XML document.

Expression	Description
Nodename	Selects all child nodes of the named node
/	Selects from the root node
//	Selects nodes in the document from the current node that match the selection no matter where they are
.	Selects the current node
..	Selects the parent of the current node
@	Selects attributes
*	Matches any element node
@*	Matches any attribute node
node()	Matches any node of any kind

Following are the XPath axes expressions which can be used to search the XML document hierarchically. This search is relative to the current node in the XML document as it sounds from there names. When we are traversing the document we can use these expressions to get the required elements.

XPath Axes	Description
ancestor	Selects all ancestors (parent, grandparent, etc.) of the current node
ancestor-or-self	Selects all ancestors (parent, grandparent, etc.) of the current node and the current node itself
attribute	Selects all attributes of the current node
child	Selects all children of the current node
descendant	Selects all descendants (children, grandchildren, etc.) of the current node
descendant-or-self	Selects all descendants (children, grandchildren, etc.) of the current node and the current node itself
following	Selects everything in the document after the closing tag of the current node
following-sibling	Selects all siblings after the current node
namespace	Selects all namespace nodes of the current node
parent	Selects the parent of the current node
preceding	Selects everything in the document that is before the start tag of the current node
preceding-sibling	Selects all siblings before the current node
self	Selects the current node

Following is the detailed XPath document from W3C Schools.
http://www.w3schools.com/xpath/xpath_intro.asp

Thanks
Prasad