Drupal's phptemplate engine has several template files and each file has several available variables. They are generally used printing their values. But, if you are bit more tricky, you can also utilize their values in various different approaches.
Viewing available variables:
There is a very simple PHP function to print available variables.
Code is:
Simple use it in your template file in which you want to see variables. For an instance, I did it on node.tpl.php and I got following variables: