As per the w3schools.com definition :
XPath is a major element in the XSLT standard. XPath can be used to navigate through elements and attributes in an XML document. XPath is a syntax for defining parts of an XML document. XPath uses path expressions to navigate in XML documents. XPath contains a library of standard functions.
w3schools.com tutorial is here.
The XPath syntax is covered here
Wikipedia defines and explains XPATH pretty nicely for a beginner.
Other interesting References and Links :
- http://blog.datahut.co/how-xpath-plays-vital-role-in-web-scraping/
- http://blog.datahut.co/how-xpath-plays-vital-role-in-web-scraping-part-2/
scrapy.org covers using XPath selectors with scrapy very nicely.