Joomla provides you with the flexibility to apply and manage templates for both the front end of joomla and the back end administration. A joomla template consists of the following key files which are required for the template to function within the context of joomla.
1) Index.php
The index.php file is the core template file and bascially defines the template layout based on module positions..
2) Template_css.css
The CSS (cascading style sheet) file defines the template format based on text , colour, tables and so on.
3) TemplateDetails.xml
TemplateDetails is a an xml which is required for the template installation plus defines the template structure, template name, author, creation date, author  URL, template description. The templatedetails css file is stored in a separate directory called CSS.
4) template_thumbnail.png
The png file is used to display the a screen  shot of the template which is visible from the  joomla administrator console.
5) Template directory
The template also contains two directories called ( css ) store's the template_css.css file and image which store the images relating to the template