Absolute Xpath: It contains the complete path from the Root Element to the desire element.
Relative Xpath: This is more like starting simply by referencing the element you want and go from the particular location.
You use always the Relative Path for testing of an element. The reason behind that is if you make any architectural change in the website the change won't effect the testing or selecting of the element.
Hope this information was what you were looking for. For more details about test automation strategies and methodology, You can get a better understanding with the Automation testing training.