Hey Karthik,
XPath uses path expressions to select nodes in an XML/HTML document. The node is selected by following a path or steps. 
The useful path expressions are listed below:
| Expression | 
Description | 
| nodename | 
Selects all nodes with the name "nodename" | 
| / | 
Selects from the root node | 
| // | 
Selects nodes in the document from the current node that matches the selection no matter where they are | 
| . | 
Selects the current node | 
| .. | 
Selects the parent of the current node | 
| @ | 
Selects attributes | 
 Example - 
/div/span/#id_name/.class_name
You can find examples and explanation here,