WordPress Category templates are very easy to use, view the WordPress Codex – Category Templates and you will see that hierarchy for a category templates is:
- 1. category-6.php
- 2. category.php
- 3. archive.php
- 4. index.php
This basically just means that WordPress will first look for a template labeled category-6, if there is none, it will then look for category.php and so forth all the way down to index.php.
More info at GabeDiaz