‘index.php’ File: 

‘index.php’ files are those file where the entire markup goes in which you stick the Joomla includes. These can be seen as little hooks where joomla hangs up information on, like modules for example.

‘templateDetails.xml’:

‘templateDetails.xml’ files are those file where you can see template details as a list of instructions to Joomla. This list must include the name of the template, the names of the files used in the template (images etc.) and the positions you want to use in your website.